Abstract:
A device and a method are used for measuring the surface topography and a wave aberration of a lens system. The device is fitted with a first measuring system containing a light source radiating a first light beam of a first wavelength, and a detector which captures the first light beam which is reflected on the lens system. In addition the device has a second measuring system containing a light source for radiating a second light beam of a second wavelength and a detector for capturing the second light beam transmitted by the lens system. A diffractive optical element is disposed in a common beam path of the first measuring system and second measuring system. The optical element adapts the respective wave-front course of the first light beam and the second light beam in a wavelength-selective manner.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is a continuation, under 35 U.S.C. § 120, of copending international application No. PCT/EP2004/009921, filed Sep. 6, 2004, which designated the United States; this application also claims the priority, under 35 U.S.C. § 119, of German patent application No. 103 42 175.0, filed Sep. 12, 2003; the prior applications are herewith incorporated by reference in their entirety. 

   BACKGROUND OF THE INVENTION 
   Field of the Invention 
   The invention relates to a device for measuring a surface topography (“topography” for short below) and wave aberration of a lens system. The device has a first measuring system that contains a light source for emitting a first light bundle of a first wavelength, a detector for picking up the first light bundle reflected at the lens system, a second measuring system that contains a light source for emitting a second light bundle of a second wavelength and a detector for picking up the second light bundle transmitted through the lens system. Such a device can be used in principle for measuring any type of optical lens system. Such a device is used, however, in particular in optical medical technology. The lens system to be examined is in this case an eye (a human one, in particular). The invention relates furthermore to an associated method. 
   The ablation of cornea tissue by excimer lasers has recently been used increasingly for operations on the human eye such as, for example, the correction of defective vision of the eye. For this purpose, a flap-like layer of the cornea is cut open and folded to the side. Thereupon a suitable amount of tissue is ablated (that is to say removed) at the cornea tissue lying open, and the cornea flap is replaced again thereafter. In this way, the cornea is specifically deformed in such a way as to compensate for defective vision of the eye, for example near or far sightedness or astigmatism. In order to be able to calculate sufficiently accurately the quantity of cornea tissue to be removed, there is a need for detailed knowledge both of the wave aberration, that is to say the defective optical imaging of the eye, and of the topography of the cornea. 
   A similar information density is also required for other correction methods on the human eye, for example the transplantation of the cornea, the exchange of the eye lens for an artificial lens, or the adaptation of a contact lens. 
   To date, the topography of the cornea and the wave aberration of the eye have usually been measured separately. This frequently gives rise to discrepancies between the two measurements carried out in temporal sequence, because of the instability of the eye as a biological object, on the one hand, and because of the numerous degrees of freedom of adjustment of the eye relative to the measuring device, on the other hand. For application in optical medicine, such a measuring discrepancy can, in particular, impair the success of an operation on the eye, or of some other medical correction method. 
   In order to avoid such discrepancies, it is therefore desirable to measure the topography and the wave aberration simultaneously. This is enabled per se in a measuring method known from U.S. patent disclosure No. 2002/0163623 A1. According to the known method and the associated device, two measuring systems are provided that emit light signals at different wavelengths and capture them again after reflection at the eye or at the cornea. In the known device, it is possible to implement the beam guidance of the light bundles required for both measurements only with a comparatively large outlay. This conversely limits the precision of the measurements. 
   A similar measuring method is further disclosed in U.S. patent disclosure No. 2001/0016695 A1. 
   SUMMARY OF THE INVENTION 
   It is accordingly an object of the invention to provide a device and a method for measuring the surface topography and wave aberration of a lens system, that overcomes the above-mentioned disadvantages of the prior art devices and methods of this general type. 
   With the foregoing and other objects in view there is provided, in accordance with the invention, a device for measuring a topography and a wave aberration of a lens systems. The device contains a first measuring system having a first light source for emitting a first light bundle of a first wavelength and a first detector for picking up the first light bundle reflected at the lens system. The device further contains a second measuring system having a second light source for emitting a second light bundle of a second wavelength and a second detector for picking up the second light bundle transmitted through the lens system. A diffractive optical element is disposed in a common beam path region of the first and second measuring systems. The diffractive optical element adapts a respective wave-front profile of the first light bundle and of the second light bundle in a wavelength-selective fashion. 
   Accordingly, there is disposed in the common beam path region of a first and of a second measuring system the diffractive optical element (that is to say one that causes light diffraction) that adapts the respective wave-front profile of a first light bundle and of a second light bundle in a wavelength-selective fashion. 
   The term wave-front profile is used for the function of the three-dimensional space that describes the spatial alignment of a surface perpendicular to the local direction of light propagation at every location within the beam path of the respectively considered light bundle. The diffractive optical element (DOE) acts in a wavelength-selective fashion to the extent that the wave-front profile of the two light bundles is influenced in a different way because of their different wavelengths when traversing the DOE. 
   The use of the DOE and of the wavelength-selective beam guidance enabled thereby permits the wave-front profile of the two light bundles to be adapted flexibly, quasi-independently and with high precision to the requirements of the respective measurement. 
   In a preferred embodiment of the invention, the DOE is configured in such a way that the zeroth diffraction order of the first light bundle is suppressed, that is to say its intensity is completely nullified or at least greatly reduced. The light of the first light bundle is, on the contrary, predominantly or completely transmitted into the first diffraction order. At the same time, as a result of a suitable configuration of the DOE, the latter exerts in essence no diffracting action on the light of the second light bundle. The second light bundle is therefore transmitted substantially unattenuated into the zeroth diffraction order. 
   By a suitable configuration of the DOE, the wave-front profile of the first light bundle transmitted by the DOE in the direction of the lens system is expediently preadapted to the topography of the lens system. Preadaptation is understood, in particular, to mean that the wave-front profile of the first light bundle is deformed by the DOE in such a way that the curvature of the wave-fronts at the location of the lens system correspond approximately to its surface curvature. This has the advantage that the light of the first light bundle impinges on the surface of the lens system in a substantially perpendicular fashion overall, and is approximately retroreflected into itself. In this way, even slight deviations in the topography of the lens system from the preadaptation can be detected precisely. Moreover, the preadaptation of the wave-front profile reduces the light loss through scattered light. When applying the invention to the human eye as lens system to be tested, the DOE is preferably fashioned in such a way that the wave-front profile of the first light bundle is adapted to a medical standard model of the human eye, in particular the Gullstrand normal eye. With regard to the second light bundle, by contrast, the DOE is preferably configured in such a way that its wave-front profile is substantially unmodified. 
   A surface-corrugated phase element is particularly suitable as DOE. This is understood to be a plate made from glass or a transparent plastic into the surface of which a relief-type diffraction grating is introduced. Such a phase element can nowadays be produced with extremely high precision in a comparatively cost-effective way by computer-aided production methods and suitable etching techniques. The diffraction action of the phase element can thereby be adapted in a highly flexible way to what is needed. It is possible in particular to use a surface-corrected phase element to achieve a surface grating with an extremely small grating period of the order of magnitude of a few hundred nanometers, and thus a comparatively large deflection angle of the diffracted light. However, it is conceivable in principle to implement the DOE in another way, for example by a volume hologram or a reflective diffractive element. In particular, it is also conceivable to configure the DOE as a flexible optical element that can be freely driven. This is possible, for example, by a phase-shifting liquid crystal displays (LCD). 
   In order to fix the refractive power and the alignment of an eye to be examined during the duration of the measurement, and thus to increase the measuring accuracy, it is preferably provided to insert a fixation target into the eye by a third light bundle. A fixation target is understood as an image that is offered for viewing to a test subject to be examined during measurement. By aiming at the fixation target, the test subject automatically keeps constant to a good approximation both the orientation of the eye and the refractive power set by the eye. The third light bundle used for inserting the fixation target has a third wavelength for which the DOE is preferably likewise inactive. The third wavelength is preferably different both from the first wavelength and the second wavelength. It is ensured in this way that the third light bundle influences the measurement of neither the topography nor the wave aberration. In a simplified configuration of the inventive device, however, it is provided as an alternative that the third wavelength corresponds to the second wavelength. 
   The two measuring systems share a common detector in a particularly rational variant of the device. 
   In order to achieve a large diffraction angle and to prevent the light bundle from dazzling the test subject to be examined during the measurement, the wavelengths of the first and/or second light bundle(s) preferably lie in the long wave and near infrared region, not visible to the human eye, of the electromagnetic spectrum. The first wavelength is selected, in particular, approximately in the range between 1000 nm and 1600 nm. The second wavelength preferably corresponds to a coarse approximation to half the first wavelength. The second wavelength correspondingly lies in the boundary region between the visible nd green spectral region and the near infrared region. 
   In order to avoid measuring errors caused by a maladjustment of the lens system with reference to the DOE, the device expediently contains an adjusting configuration with the aid of which the position of the lens system can be determined and set. A particularly advantageous implementation of such an adjusting configuration contains a light source and a position-sensitive detector. In order to adjust the lens system, an adjusting light beam is cast by the light source onto the lens system at an angle, and the adjusting light beam reflected thereat is captured on the position-sensitive detector. In order to adjust the lens system, the position of the lens system is now varied with reference to the DOE until the reflected adjusting light beam impinges at a predetermined point of the detector surface that indicates the correct adjustment of the lens system with reference to the DOE. 
   It is advantageous for a particularly precise adaptation of the wave-front profile of the light bundles when the DOE is directly upstream of the lens system such that the light of the light bundle emanating from the DOE falls directly onto the lens system. Conversely, the light of the light bundle that is retroreflected by the lens system again falls directly onto the DOE before it is led to the detectors of the two measuring systems. 
   As a result of a wavelength-selective beam splitter that limits the common beam path region of the two measuring systems, the two light bundles are fed effectively to the respective detector after reflection at the eye. As a result, an undesired interaction of the two measuring systems that could lead to a falsification of the measurement result is avoided, in particular. In an expedient embodiment of the invention, such a beam splitter is configured, in turn, as a diffractive optical element that separates the beam path of the two light bundles from one another in a wavelength-selective fashion. 
   A so-called wave-front detector is suitable as the detector of the first and/or the second measuring system. This is understood as a detector that detects the spatial alignment of a wave-front. The use in particular of a Shack-Hartmann sensor or an interferometer, for example of shearing type, as the detector of the first and/or second measuring system is provided within the scope of the device according to the invention. A pyramidal sensor or a Talbot interferometer can, furthermore, advantageously be used within the scope of the device according to the invention. 
   In the course of the method carried out with the aid of the device according to the invention, it is preferred to measure the topography and the wave aberration simultaneously. A particularly short measuring time is thereby achieved. This, in turn, is of advantage in particular when applying the method to the human eye, all the more so as a test subject to be examined has to keep still as much as possible during the measurement, and this necessarily entails a certain unpleasantness. On the other hand, the simultaneous measurement of topography and wave aberration prevents discrepancies between the respective result of these two measuring methods which would occur owing to the instability of the human eye for measurements separated in time. 
   A temporally sequential measurement, that is to say one that is offset in time (in particular at a very short interval) is, nevertheless, regarded as an advantageous alternative with regard to a simplified conduct of the method. Particularly when use is made of a common detector for both measuring systems, this is sensible in order to separate the measuring signals of the two measuring systems more effectively. In order to avoid measuring discrepancies, the measurements are preferably carried out in a time interval that is less than the reaction time of the eye so that the measurements is conducted quasi-simultaneously with regard to the eye. 
   In an expedient alternative embodiment of the method according to the invention, the topography and/or the wave aberration are/is measured with the aid of a so-called scanning spot method. In this inherently conventional measuring technique, the lens system is not illuminated with a flat light bundle, but is scanned with a fine light beam of approximately punctiform cross section. Accordingly, in the case of a scanning spot method it is not the wave-front profile of the light reflected by the lens system that is measured, but the deflection of the reflected thin light beam from a desired position characteristic of an “ideal lens system”. A customary wave-front method and a scanning spot method are equivalent with regard to the information content relating to the topography or the wave aberration of the lens system being examined. 
   Other features which are considered as characteristic for the invention are set forth in the appended claims. 
   Although the invention is illustrated and described herein as embodied in a device and a method for measuring the surface topography and wave aberration of a lens system, in particular an eye, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
   The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagrammatic illustration of a device for measuring a topography and wave aberration of a lens system, in particular the human eye, having a diffractive optical element (DOE) disposed in a common beam path region of a first measuring system and a second measuring system according to the invention; 
       FIG. 2  is a diagrammatic, cross-sectional view showing the diffractive optical element in accordance with  FIG. 1 ; 
       FIG. 3  is a diagrammatic, sectional view of the diffractive optical element and the eye in the beam path of a first light bundle for measuring the topography of the eye; 
       FIG. 4  is a diagrammatic, sectional view of the diffractive optical element and the eye in the beam path of a second light bundle for measuring the wave aberration of the eye, in an illustration in accordance with  FIG. 3 ; 
       FIG. 5  is a diagrammatic, illustration of an alternative configuration of the device in an illustration in accordance with  FIG. 1 ; and 
       FIGS. 6 to 8  are diagrammatic, illustrations showing further embodiments of the device in an illustration in accordance with  FIG. 1 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In all the figures of the drawing, sub-features and integral parts that correspond to one another bear the same reference symbol in each case. Referring now to the figures of the drawing in detail and first, particularly, to  FIG. 1  thereof, there is shown a schematic illustration of a device  1  for measuring the topography and wave aberration of a lens system. The term lens system here includes any artificial or natural optical system equipped with one or more lenses. The lens system illustrated schematically in  FIG. 1  is, in particular, a human eye  2 . 
   The three-dimensional shape of the lens surface is denoted as (surface) topography. In the case of the eye  2 , a lens surface is a surface  3  of a cornea  4 . The lens system of the eye  2  further contains in a known way an eye lens  5  and a vitreous body  6 . A retina  7  is disposed in a known way in the fundus of the eye opposite the eye lens  5 . 
   The term wave aberration denotes in general the deviation of the optical imaging properties of the real lens system to be tested from the imaging properties of a corresponding ideal lens system. In the case of the eye  2 , the wave aberration contains first order aberrations such as near sightedness, far sightedness or astigmatism, as well as higher order aberrations. 
   The device  1  is provided with a first measuring system  8  for measuring the topography of the cornea  4 . A second measuring system  9  is provided for measuring the wave aberration. 
   The first measuring system  8  contains a light source  10 , in particular a laser. The light source  10  produces a first light bundle  11  of a first wavelength λ1. The light bundle  11  is first directed in parallel in a collimator lens  12  along the beam path of the first measuring system  8  and irradiated into a common beam path region  14  of the measuring systems  8  and  9  by a wavelength-selective beam splitter  13 . Within the beam path region  14 , the first light bundle  11  is expanded by a Kepler telescope  17  formed of two lenses  15  and  16 , and traverses a diffractive optical element (DOE)  18  directly upstream of the eye  2 . The first light bundle  11  is collimated in the direction of the eye  2  by the DOE  18 , whose mode of operation is described in more detail below. A component of the light bundle  11  incident on the eye  2  (denoted below as reflected light bundle  11 ′ for the sake of simplicity) is reflected at the surface  3  of the cornea  4  and retroreflected against the direction of incidence through the DOE  18 , the Kepler telescope  17  and the beam splitter  13 . The reflected light bundle  11 ′ is coupled out of the incident light bundle  11  and led to a wave-front detector  20  by a further beam splitter  19  disposed outside the common beam path region  14 . The Kepler telescope  17  is configured here in such a way that the cornea  4  is imaged sharply on the wave-front detector  20 . The wave-front detector  20  is optionally configured as a Shack-Hartmann sensor, as is described in U.S. patent disclosure No. 2003/0038921 A1, for example. Alternatively, the wave-front detector  20  can also be configured as an interferometer, in particular a shearing interferometer. 
   The second measuring system  9 , provided for measuring the wave aberration, contains a further light source  21 . The light source  21 , which is preferably implemented, in turn, by a laser, emits a second light bundle  22  of a second wavelength λ2 in the form of a comparatively fine light bundle. The second light bundle  22  is once again directed in parallel in a collimator lens  12  and irradiated through the wavelength-selective beam splitter  13  into the common beam path region  14 . Because of its wavelength selectivity, the beam splitter  13  is transparent to the wavelength λ2, and therefore inactive. A beam splitter  13  having such wavelength selectivity can be produced according to a conventional technique by a dielectric mirror, for example. 
   In the further course of its beam path, the second light bundle  22  falls through the Kepler telescope  17  and the DOE  18  onto the eye  2 . In a manner described in more detail below, the DOE  18  is thereby fashioned in such a way that it has no diffracting action, or only a negligible one, on light of wavelength λ2. The light bundle  22  consequently traverses the DOE  18  in a quasi-unmodified fashion and falls onto the retina  7  as a further fine light bundle through the cornea  4  and the eye lens  5 . The light bundle  22  is backscattered diffusely at the retina  7 . The scattered light, denoted below as backscattered light bundle  22 ′, falls back against its direction of incidence through the eye lens  5 , the cornea  4 , the DOE  18 , the Kepler telescope  17  and the beam splitter  13  transparent to the wavelength λ2. A further beam splitter  23  positioned outside the common beam path region  14  in the beam path of the light bundle  22 ,  22 ′ couples out the backscattered light bundle  22 ′ and casts it onto a wave-front detector  24  of the second measuring system  9 . The wave-front detector  24  is, in turn, optionally configured as a Shack-Hartmann sensor or as an interferometer. A precompensation unit  25  is interposed between the beam splitters  13  and  23 . The precompensation unit  25  includes a conventional non-illustrated optical zoom system or a lens configuration with the aid of which it is possible to compensate the components of defocus and astigmatism, that is to say the near or far sightedness and astigmatism. The precompensation unit  25  serves conversely also for the purpose of imaging the incident light bundle  22  sharply on the retina  7 . 
   Other than in the case of the eye  2 , the wave aberration can be measured in general in a simplified fashion for an artificial lens system by disposing the lens system between the light source and detector such that the lens system is transluminated only once by the second light bundle. 
   The DOE  18  illustrated in  FIG. 1  is a so-called surface-corrugated phase element whose configuration and mode of operation are sketched schematically in  FIG. 2 . The DOE  18  depicted is a platelet made from glass or a transparent plastic and into a surface  26  of which facing the eye  2  there is introduced a relief-type diffraction grating. The diffraction grating of the DOE  18  contains a number of depressions  27  that are approximately of annular shape and disposed approximately concentrically about the optical axis  28  ( FIG. 1 ) of the device  1 , and are separated from one another by interposed webs  29 . The depressions  27  and the interposed webs  29  can deviate in a predetermined way from the spherical shape and concentric configuration in order to achieve aspheric diffraction patterns. 
   As is to be seen from  FIG. 2 , preferably all the depressions  27  have the same structural depth h such that the surface  26  is subdivided into two discrete levels. A DOE configured in such a way is also denoted as a binary hologram. 
   With regard to their optical action, the depressions  27  and webs  29  of the relief-type surface  26  form alternating phases within which an incident plane light wave L develops differently as a consequence of the difference in refractive index between the material of the DOE  18  and the ambient air. As a result, upon traversal of the light wave L through the surface  26 , a phase difference arises between partial waves in the region of the depressions  27  and partial waves in the region of the webs  29  that lead in a way known per se to interference effects and diffraction effects. 
   These effects have the consequence that when exiting from the DOE  18  the irradiated light wave L emits only in discrete directions with reference to the direction of incidence  30 . The partial beams emitted in these discrete directions are denoted as diffraction orders  31 ,  32 ,  33 . In general, a portion of the irradiated intensity of radiation is emitted without diffraction, that is to say in the direction of incidence  30 . This partial radiation is denoted as zeroth diffraction order  31 . The further diffraction maxima are denoted as first diffraction order  32 , second diffraction order  33 , etc. as angle increases with reference to this zeroth diffraction order  31 . For reasons of simplicity, higher diffraction orders are not illustrated in the schematic in accordance with  FIG. 2 . 
   The angles at which the individual diffraction orders  32 ,  33 , etc. appear with reference to the zeroth diffraction order  31  are a function of the grating constant of the diffraction grating, that is to say the spacing between two neighboring depressions  27 . The diffraction angles are larger here the smaller the grating constant. 
   It can be disposed for the irradiated light to be preferably emitted into a specific diffraction order  31 ,  32  or  33  by a suitable configuration of the DOE  18  with regard to the wavelength of the irradiated light wave L. This is utilized according to the invention by tuning the diffraction grating of the DOE  18  with wavelengths λ1 and λ2 for wavelength-selective adaptation of the wave-front profile of the light bundles  11  and  22 . 
   Here, the DOE  18  is tuned to the wavelength λ1 in such a way that partial waves of the light bundle  11  in the region of the depression  27  on the one hand, and in the region of the web  29 , on the other hand, pierce the surface  26  precisely in a fashion offset by an odd multiple of the half wavelength, and thus interfere negatively in the direction of incidence  30 , that is to say along the optical axis  38 . This condition is fulfilled whenever the structural depth h satisfies the equation 
                   h   =     i   ·       λ   1       2   ·     (       n   ⁡     (     λ   1     )       -   1     )             ,           Equation   ⁢           ⁢   1               
where n(λ 1 ) being the refractive index of the material of the DOE  18  for the wavelength λ 1  and i=1,3,5 . . . being an odd, natural number.
 
   Furthermore, the DOE  18  is fashioned such that the surface area occupied by the depression  27  corresponds substantially to the surface area of an adjoining web  29 . As a result of this, the zeroth diffraction order  31  of the first light bundle  11  is suppressed almost completely by destructive interference. 
   As is to be seen from  FIG. 3 , when the light bundle  11  illuminates the DOE  18 , a curved wave-front profile  34  is formed in the region of the eye  2  owing to the previously described diffraction effect. By suitably varying the grating spacing of the DOE  18 , the wave-front profile  34  is preadapted to the eye  2  such that the curvature of the wave-fronts  35  in the region of the cornea  4  corresponds to the average surface curvature of the human cornea. The DOE  18  is optionally formed in such a way that the curved wave-front profile  34  corresponds to a spherical wave. A spherical wave includes, in particular, a focus with the aid of which the entire measuring system  8  can be calibrated in a comparatively simply way. In order to preadapt an aspheric deviation of the cornea surface with comparatively simple measures, the DOE  18  is alternatively formed in such a way that the curved wave-front profile  34  corresponds to a spherical wave with a conical component adapted to the cornea  4 . Alternatively, it is provided in turn for the DOE  18  to be formed in such a way that the shape of the wave-fronts  35  at the location of the cornea  4  corresponds to the average shape of the cornea  4  to be derived from the standard eye model of Gullstrand. 
   Were the topography of the cornea  4  to correspond exactly to the preadaptation, in particular to the curvature described by the Gullstrand normal eye, the light bundle  11  would impinge exactly vertically on the surface  3  of the cornea  4  and be retroreflected exactly into itself onto the DOE  18 . In reversal of the above-described diffraction effect, in this ideal case the DOE  18  would transform the curved wave-front profile of the reflected light bundle  11 ′ into a plane wave that would correspond exactly to the incident light bundle  11 . 
   The topography of the cornea  4  of the real eye  2  is, however, individually different and, in particular, deviates more or less strongly from the Gullstrand normal eye. Consequently, the wave-front profile  34  of the light bundle  11  is distorted upon reflection at the surface  3 . Consequently, upon traversing the DOE  18  the reflected light bundle  11 ′ is converted into a wave-front  36  that is curved by comparison with the incident plane wave-front. The curvature of the wave-front  36  is detected in the wave-front detector  20 . The topography of the cornea  4  can be calculated therefrom with the aid of computational methods known per se. 
   On the other hand, the DOE  18  is tuned to the wavelength λ2 in such a way that the second light bundle  22  is preferably transmitted into the zeroth diffraction order  31 . This is always the case when a phase difference that is an integral multiple of the full wavelength λ2 is reached between the depressions  27  and adjacent webs  29 . The condition for this is that the structural depth of the equation 
                 h   =     j   ·       λ   2       (       n   ⁡     (     λ   2     )       -   1     )                 Equation   ⁢           ⁢   2               
is sufficient, n(λ 2 ) being the refractive index of the material of the DOE  18  for the wavelength λ2, and j=1,2,3, . . . being a natural number.
 
   As may be seen from  FIG. 4 , if this condition is fulfilled the wave-front profile  34  of the light bundle  22  is virtually not modified during passage through the DOE  18 . The DOE  18  is therefore substantially inactive for the light bundle  22 . 
   The measurement of the wave aberration of the eye is performed in accordance with  FIG. 4  in such a way that an approximately punctiform spot  37  is illuminated on the retina  7  with the aid of the light bundle  22  formed as a fine beam. The light bundle  22 ′ diffusely backscattered by the spot falls back through the eye lens  5  and the cornea  4  onto the DOE  18  and is transmitted from there in the direction of the wave-front detector  24 . 
   As a consequence of its slight beam cross section, the incident light bundle  22  is influenced by the wave aberration of the eye  2  only to a comparatively slight extent. However, on the return path the backscattered light bundle  22 ′ penetrates the full cross-sectional surface of the eye pupil and consequently collects the entire information relating to the wave aberration of the eye  2 . 
   Whereas in the case of an ideal imaging characteristic of the eye  2 , that is to say given a vanishing wave aberration, it would be expected that the light bundle  22 ′ emanating from the illuminated spot  37  as a quasi punctiform light source ought to be imaged by the (relaxed) eye  2  into a plane wave with parallel wave-fronts, the wave-fronts  36  of the light bundle  22 ′ are curved, very much as a rule, by the non-vanishing wave aberration of the real eye  2 . This curvature is precompensated in a first order (by correcting near sightedness, far sightedness or astigmatism) in the precompensation unit  25 . The higher order curvature, remaining after the precompensation, of the wave-front  36  is detected by the wave-front detector  24 . Methods known per se are used to calculate the wave aberration of the eye  2  therefrom. 
   The wavelengths λ1 and λ2 are selected for the purpose of achieving larger diffraction angles, preferably in the comparatively longwave infrared region. The invisible infrared light has the advantage, in addition, that both the topography and the wave aberration can be measured without this being noticed by the test subject. Consequently, in particular, the test subject is prevented from being dazzled by the light bundles  11  and  22 , or from reacting to the irradiation of light in a way that impairs the measurement. Advantageous wavelength combinations are, in particular, λ1=1550 nm and λ2=785 nm. On the other hand, from the point of view of better availability of sensitive and comparatively cost-effective light sensors, the selection of smaller wavelengths for the first light bundle  11  and/or the second light bundle  22  is also advantageous; in particular, λ1=1064 nm and/or λ2=532 nm. It is preferred to measure the topography and the wave aberration simultaneously, but the measurements can also be carried out individually or sequentially with a time offset. 
     FIG. 5  shows an alternative embodiment of the device  1 . This embodiment differs from the configuration in accordance with  FIG. 1  in that here the beam splitter  13  directly follows the DOE  18 . Therefore, it is not until immediately before they fall onto the DOE  18  that the light bundles  11  and  22  are combined in the common beam path region  14  and the light bundles  11 ′,  22 ′ are separated on the return path directly after traversing the DOE  18 . For the purpose of expanding or imaging the light bundles  11 ,  11 ′ and  22 ,  22 ′, one Kepler telescope  17  each is disposed outside the common beam path  14  in the beam path of the two light bundles  11 ,  11 ′ and  22 ,  22 ′. 
   In the configuration in accordance with  FIG. 5 , the device  1  contains a third light source  38  by which a third light bundle  39  of a third wavelength λ3 can be inserted into the eye  2 . The third light bundle  39  is, in turn, directed in parallel by a collimator lens  40  and aligned by a wavelength-selective beam splitter  41  with the optical axis  29  and thus with the eye  2 . The third light bundle  39  serves the purpose of offering the eye  2  a so-called fixation target. This is understood as an image at which the test subject aims during the measurement. First, aiming for the fixation target results in that the viewing axis of the eye  2  is aligned along the optical axis  28 . Second, the refractive power of the eye lens  5  is fixed in a range in which the test subject can sharply detect the fixation target. In particular an image at infinity is frequently simulated for the test subject by the fixation target such that the eye lens  5  is held in the relaxed state during the measurement. The third light bundle  39  likewise traverses the precompensation unit  25  in order, in particular, to compensate any possible shortsightedness of the eye  2 , and so to give the test subject the actual possibility of sharply aiming at the fixation target. The wavelength λ3 of the third light bundle  39  must necessarily be in the visible spectral region, and is preferably selected in such a way that the DOE  18  does not exert any diffracting action on the light bundle  39 . So as to simplify the measuring setup, the third wavelength λ3 can therefore also be selected, in particular, to be equal to the second wavelength λ2. In this case, the third light bundle  39  is blocked out briefly during the measurement of the wave aberration. Alternatively, the third wavelength λ3 can also, however, be selected in such a way that the DOE  18  suppresses the zeroth diffraction order  31  of the third light bundle  39 . 
   So-called maladjusting aberrations can also be caused by a false positioning of the eye  2  with reference to its lateral and axial position in relation to the DOE  18 . The eye  2  must therefore be adjusted exactly with reference to the device  1  before measurement is begun. In a way similar to conventional measuring devices of optical medicine, the test subject is fixed for the measurement with reference to the device  1  by bearing surfaces (not illustrated in more detail) for chin and forehead. Since the dimensions of the head vary from test subject to test subject, however, a fine adjustment of the device  1  with reference to the fixed head of the test subject is required for correct positioning of the eye  2 . An adjusting configuration  42  that includes a light source  43  and a position-sensitive detector  44  and is illustrated schematically in  FIG. 5  serves the purpose of facilitating this adjusting operation. The light source  43 , which is, in particular, a laser, casts obliquely onto the eye  2  a fine adjusting light beam  45  that is reflected at the cornea  4  in the direction of the position-sensitive detector  44 . With correct adjustment of the eye  2 , the adjusting light beam  45  impinges at a predetermined point on the detector  44 . By contrast, if the eye  2  is maladjusted, the reflected adjusting light beam  45  impinges at a point differing therefrom or—given coarse maladjustment of the eye  2 —does not impinge on the detector  44  at all. Before measurement is started, the position of the device  1  is therefore adjusted with reference to the eye  2  until the point of impingement of the reflected adjusting light beam  45  corresponds to the predetermined point  46  on the detector  44 . In the simplest case, the detector  44  is a viewing screen on which the point of impingement of the adjusting light beam can be observed by eye. However, the detector  44  can also be an electronic detector, in particular a CCD sensor. 
   A further embodiment, illustrated in  FIG. 6 , of the device  1  differs from the above-described configurations in that the wavelength-selective beam splitter  13  is likewise configured as a diffractive optical element. In the same way as the DOE  18 , the beam splitter  13  in this case selectively diffracts the wavelength λ1, while light of wavelength λ2, in particular the light bundle  22 , transmits the beam splitter  13  undiffracted. The beam splitter  13  has a surface structure or volume structure in the form of a regular linear grating, as a result of which the beam path of the first light bundle  11 ,  11 ′ is deflected by a defined angle. The beam splitter  13  and the DOE  18  can thereby also be united in a single optical element. The beam path of the light bundle  11 ,  11 ′ is deflected again into a direction parallel to the optical axis  28  by a further DOE  47 . By contrast with the beam splitter  13  and the DOE  18 , the DOE  47  is hereby configured as a reflective element, in particular as a sawtooth prism. 
   A common detector  48  for both measuring systems  8  and  9  is provided in the further variants of the device  1  that are illustrated in  FIGS. 7 and 8 . This particularly rational configuration is rendered possible by the fact that, after being coupled out of the optical axis  28  by a mirror  49 , the backscattered second light bundle  22 ′ is deflected in the direction of the beam splitter  19  of the first measuring system  8 . In a way similar to the beam splitter  13 , in this configuration the beam splitter  19  is of frequency-selective design and to that extent transparent to the wavelength λ2, in order to ensure a quasi unattenuated transmission of the light bundle  22 ′ onto the detector  48 . 
   The Kepler telescope  17  disposed in accordance with  FIG. 7  in the beam path of the second light bundle  22 ′ fulfills the additional task of expanding the light bundle  22 ′ in order to utilize the detection surface of the detector  48 , and thus to exploit the resolution of the detector  48 . The same goal is served by a further Kepler telescope  50  additionally disposed in accordance with  FIG. 8  in the beam path of the second light bundle  22 ′. 
   In order to be able to separate from one another the signal components, detected by a common detector  48 , of the two measuring systems  8  and  9 , the measurements of topography and wave aberration are preferably carried out sequentially in time. The two measurements are performed in this case shortly after one another in such a way that the eye  2  remains quasi static during the entire measurement period. The measurements of topography and wave aberration are therefore performed quasi-simultaneously on the time scale of a typical reaction time of the eye  2 . A simultaneous measurement of topography and wave aberration is, however, also possible with the aid of the common detector  48  to the extent that the latter includes a color-sensitive sensor, for example an RGB sensor. The signal components of the two measuring systems  8  and  9  can then be separated after the detection as a consequence of the different wavelength λ1 or λ2 of the two light bundles  11 ′ and  22 ′.