DE102008008732A1 - Method and device for determining corneal radii - Google Patents

Abstract

The invention relates to a method and arrangements for determining the inner corneal radius. In the process, a pattern of known geometry is projected onto the cornea and the light reflected from the front of the cornea and the back of the cornea is analyzed.

Description

The The invention relates to a method and an arrangement to optical Determination of the inner corneal radius (posterior corneal radius).

The Cornea contributes about 40% to the total refractive power of the at the optical apparatus of the eye. This is their influence in particular in the optimal determination of intraocular lenses for to consider the cataract treatment, but also for the Planning a refractive corneal correction is the knowledge of Corneal geometry required.

The front of the cornea is relatively easily accessible to a measurement, and thus the determination of the outer corneal radius by means of so-called keratometer is possible. The measuring principle is based on projecting a known pattern on the cornea and recording the image reflected by the curved surface with an image receiver arranged on the optical axis of the eye and evaluating it geometrically. The best known is the projection of a so-called Placidoscheibe, which consists of sharply defined concentric rings on the cornea. The shape of the image of the reflected rings indicates the curvature and thus the radius of the cornea. Such solutions are described for example in US 6,692,126 . US Pat. No. 6,575,573 and US 7 246 905 , In the DD 251 497 It is described that, instead of imaging of rings, discrete points can also be imaged on the cornea, and their curvature can be determined from the evaluation of the geometric locations of the reflexes of these points. In the US 5,886,767 a pattern consisting of dots is projected onto the cornea from a projector arranged on the optical axis of the eye, and the resulting scattering image is recorded with two cameras arranged at an angle to the axis of the eye and evaluated stereoscopically.

Furthermore, solutions have been described how the thickness of the cornea on the axis of the eye can be determined. Already in 1975 was from DG Green et al., "Corneal thickness measured by interferometry," Journal of the Optical Society of America, Col. 65, February 1975, p. 119 , proposed to make the measurement of the corneal thickness interferometrically. For this purpose, the light of a laser is directed to the center of the cornea so that incident and reflected light form an angle of 90 °. The light reflected at the corneal front and back interferes, the resulting interference fringes are evaluated to determine the thickness of the cornea on the axis of the eye. In "Measurement of corneal thickness by low-coherence interferometry", Ch. K. Hitzenberger, Applied Optics Vol. 31, 1992, p. 6637 Laser Doppler interferometry is used to measure central corneal thickness. M. Boehnke et al. "Real-time pachymetry during photorefractive keratectomy using optical low-coherence reflectometry", J. Biomediacal Optics Vol. 6, 2001, p. 412 ) describe the measurement of the central corneal thickness during a refractive laser treatment by means of optical short-coherence reflectometry. Optical Coherence Tomography (OCT) has also been used to determine central corneal thickness ( J. Wang et al, "Noncontact Measurement of Central Corneal Epithelial and Flap Thickness of Old Laser In Situ Keratomileusis," Investigative Ophthalmology and Visual Science, Vol. 45, 2004, p. 1812 ). In US Pat. No. 7,154,111 the determination of the central corneal thickness is described with a detector operating on the confocal principle. In US 7 133 137 an arrangement is given in which the respective interference of light of three wavelengths is used as a starting point for determining the central corneal thickness.

A determination of the inner corneal radius is not possible with these methods. To get information about the shape of the cornea, it has been known since the beginning of the 20th century, the slit lamp as a standard device. In this case, a light section through the eye is generated by means of a gap, which is viewed laterally. The cornea also forms in this light section. As a further development of the slit lamp so-called Scheimpflugkameras can be called as they are for. In DE 299 13 601 U1 . DE 299 13 602 U1 . DE 299 13 603 U1 and US Pat. No. 7,264,355 are described. In principle, information about the curvature of the cornea can be obtained by evaluating the recorded sectional images. However, this would require lateral cross-sectional images with extremely high spatial resolutions, since the resulting from the curvatures axial position changes are marginal.

Therefore is in the well-known device OrbScan (Orbtec, Bausch & Lomb, Rochester) a keratometer with Placido disc combined with a slit lamp.

It is also known to combine a keratometer with an OCT system (eg. US 7 246 905 or WO 2005/077256 ). In this case, the OCT is used to obtain a sectional view of the anterior segment of the eye, from which information about the thickness of the cornea and, given a known curvature of the corneal anterior surface, could in principle also be derived via the inner radius. However, such a device is very expensive, and again the requirements for spatial resolution are immense in order to be able to measure useful changes in curvature.

The invention is based on the object to eliminate the disadvantages of the prior art and a simple and accurate method for Bestim indication of the inner radius of the cornea.

These The object is achieved by a method for determining the inner corneal radius, wherein at least a pattern of known geometry is projected onto the cornea and each from the front of the cornea and the back of the cornea reflected light is analyzed.

A advantageous embodiment of the invention results when the of the cornea front and the cornea back reflected Light is brought to interference and the resulting interference pattern is evaluated.

A Another advantageous solution is that of the cornea front and the cornea back reflected light geometrically evaluate. In this case, this geometric evaluation according to the invention by Adjust the intensities of the front and back reflections done by polarization optical means. Another advantageous Solution arises when these reflections with an image acquisition unit be recorded with a high dynamic range.

A first inventive arrangement for implementation The procedure for determining the inner corneal radius consists of a light source having a coherence length of greater than 2 mm, preferably in the range of 2 to 6 mm, a unit for producing a pattern of known geometry, an imaging optics, which images the test pattern on the cornea, a recording optics, an image recording unit and an image evaluation unit.

there It is advantageous if the image evaluation unit for analyzing the from the anterior corneal reflected light and for analysis by the interference of the front and the back formed reflected light resulting interference pattern is.

A second inventive arrangement for implementation The procedure for determining the inner corneal radius consists of at least one light source, a unit for generating a pattern known geometry, at least one imaging optics, which the Test pattern on the cornea, a recording optics, a Image recording unit and an image evaluation unit.

there It may be advantageous if the image recording unit over has a high dynamic range.

alternative can according to the invention in the imaging beam path at least one polarizer and in the recording beam path an analyzer be arranged.

there can the pattern of known geometry from a structure of several Rings in the form of a Placido disk or discrete light sources, here preferred VCSEL sources with a wavelength between 650 and 1550 nm, exist.

at the method of determining the inner radius of the cornea with use the interference may be beneficial if the interference pattern for several phase angles is evaluated by varying the wavelength the light sources are obtained during several measurements.

One Another advantageous method for determining the inner corneal radius results when the wavelength of the light sources used is varied rapidly during a measurement to be at the geometric Evaluation of unwanted interferometric signal components to suppress.

The The invention will be described in more detail below with reference to the schematic figures explained.

It demonstrate:

1 : a first basic structure of an arrangement according to the invention

2 : a second basic structure of an arrangement according to the invention

3 : Details of the second basic structure

4a to d: different variants of polarizer / analyzer combinations of the second basic design

5 : a variant of the second basic construction with rotatable analyzer

6 : a second variant of the second basic structure

7 : a second view of the arrangement 1 or 2

8th : a third basic structure of an arrangement according to the invention

9a and b: camera images of the reflexes of the front and back of the cornea

In 1 a first embodiment of the invention is shown schematically. It shows the cornea 1 with her front 2 and back 3 and the eye lens 4 , That of a punctiform light source shown here 5 Outgoing light is at the front of the cornea 2 and the cornea back 3 reflected, the resulting reflexes 6 (Before the side reflex) and 7 (Backside reflex) are from a camera 8th recorded and with an evaluation unit 9 evaluated. A control unit 10 controls the arrangement and the measurement process.

On the representation of an imaging optics for imaging the light source 5 on the cornea 1 and a recording optics for imaging the reflections 6 . 7 to the camera 8th was omitted for the sake of simplicity, also exemplified only a light source 5 represented, the pattern of known geometry is correspondingly by a plurality of light sources 5 educated.

The method according to the invention now consists of the light sources 5 to focus on the cornea and the image of the reflexes 6 . 7 on the camera 8th analyze.

This can be done for the front reflexes 6 the known in the prior art method US 6,692,126 . US Pat. No. 6,575,573 . US 7 246 905 or DD 251 497 , the contents of which are hereby incorporated by reference.

Regarding the evaluation of the backside reflexes 7 further considerations, which are not known from the prior art, are needed.

How out 1 to see is lying front reflex 6 and backside reflex 7 on the camera picture close to each other. In addition, because of the refractive index jump on the front of the cornea 2 much higher than at the back 3 is the front reflex 6 100x stronger than the back reflex 7 Additional measures are necessary to be able to evaluate the images of the two reflections geometrically.

A first solution to this problem is the image of the reflexes 6 . 7 to record with a very highly dynamic camera, ie with a dynamic of well over 120: 1, ie 7bit to over 12bit, with about 1000: 1 or 10bit dynamics are preferred.

Examples of such cameras are either CCD cameras with a dynamic expansion algorithm or highly dynamic CMOS cameras. This also allows the very different brightnesses of the front side reflections 6 and backside reflexes 7 record and evaluate. For the evaluation of the backside reflexes 7 can use the same methods as for the front reflexes 6 to be used.

A second very advantageous solution to this problem is to adjust the intensities of the reflections 6 . 7 to achieve with polarization optical means. The invention makes use of the fact that the cornea has birefringent properties. This effect is in the US 7,287,885 , the entire contents of which are hereby incorporated by reference.

In 2 are in the basic structure after 1 polarizers 11 to polarize the light sources 5 coming light and an analyzer 12 in front of the camera 8th inserted.

At the same time, the corneal anterior reflex becomes 6 polarization optically suppressed, so as to adapt to the intensity of the backside reflection 7 to achieve, resulting in a simpler readability of the camera 8th obtained images leads. The basics for this polarization suppression are explained in more detail below.

In 2 becomes a plane between the on the cornea 1 incident beam and that from the front of the cornea 2 reflected beam spanned, the plane of incidence. A direction of polarization perpendicular to this plane is referred to as s-polarization and in-plane polarization as p-polarization. The reflection of the light sources 5 emitted beam on the cornea 1 is described via the Fresnel equations. If the polarization of the incident beam is a pure s or p polarization, then the polarization state is not changed during the reflection.

Will now the polarization of the incident wave rotationally symmetrical in the radial or in the azimuthal direction by a corresponding polarizer 11 adjusted so that the polarization state can be changed by the cornea, so can the reflex 6 from the front of the cornea 2 by an analyzer perpendicular to the polarization of the incident beam 12 be completely suppressed while reflex 7 remains detectable. In the 2 Polarizers shown may consist of an annular excitation polarizer 11 and a circular analyzer (detection polarizer) 12 consist. The excitation and detection polarizer 11 . 12 can be used as independent components ( 3 ) or as a composite polarizer. ( 4a to 4d ) Another technical design option is in 4b shown. If one uses this element, then in the method according to the invention a plurality of images, each with a rotated analyzer 12 except and assembled into an approximately polarization rotation symmetric image, which is then evaluated. Intended to be a unit for rotating the analyzer 12 it is also possible, as in 4c represented one from z. B. 16 linear polarizers composite polarizer / analyzer to use.

In this case, a crosstalk between to avoid adjacent polarizers by aberrations in the imaging optics, the edges of the polarizers are advantageously blackened.

By the in 4a to 4c illustrated polarizer arrangements 11 . 12 is achieved that the polarization of the incident light wave in the reflection on the front cornea 2 is not changed and with a perpendicular to the Anreungspolaristion detection polarizer 12 almost completely suppress. The light which enters the cornea 1 enters and on the back surface 3 On the other hand, due to the double computation, a reflected state of polarization is reflected and therefore becomes the detection polarizer 12 not completely suppressed. So it is possible the intensity ratio of corneal back reflex 7 and corneal anterior reflex 6 in the camera picture 8th to increase significantly.

there Two optimization states are possible.

On the one hand can be tried with an arrangement after 3 . 4a and 4c the corneal anterior reflex 6 completely suppress. For this purpose, a rotationally symmetric s or p polarization is irradiated. Due to this polarization symmetry in the azimuthal or radial direction, however, the form birefringence of the cornea can 1 are not exploited, since their coordinate system is also aligned in the azimuthal or radial direction. For this reason, the shape birefringence can not change the state of polarization and it also becomes the corneal backside reflex 7 strongly suppressed. Due to the residual birefringence, however, there is a slight change in polarization when passing through the cornea 1 and the backside reflex 7 is not completely suppressed. The degree of suppression, however, depends on the strength and the orientation of the residual birefringence, which for each site of the cornea 1 and can be different for each person. For this reason, it may be better to build up 4b to use, because in this structure by rotating the polarizing filter combination 11 . 12 Each polarization axis can be measured.

In the second optimization, on the other hand, you do not try the front-page reflex 6 completely suppress, but the back reflex 7 minimally suppress. A polarization filter combination that corresponds to this optimization is in 4d shown. In this construction, the diameter of the light sources 5 pattern formed so that the mean phase deviation due to the birefringence in the cornea 1 in a middle eye in the two polarization directions when passing through the cornea twice 1 is about λ / 2 and so is the backside reflection 7 the cornea almost undiminished by the detection polarizer 12 can occur. In this case, the Fresnel formulas also describe how strong the polarization on the front side 2 the cornea 1 when the reflection is changed.

Due to the reflection on the front 2 the cornea 1 the polarization of an incident wave is changed, if not purely s- or p-polarized is irradiated. Since the reflection occurs on a more dense medium with very weak absorption, the phase difference of the reflected s and p components is always 0 (or 180 ° for angles of incidence above the Brewster angle).

Thus, the reflected wave is linearly polarized, but has due to the different reflection coefficients in s and p polarization in relation to the polarization polarization changed polarization direction. In this case, in the arrangement according to 4b or 4d the orientation of the detection polarizers 12 be adjusted so that the anterior corneal reflex 6 is completely suppressed even in a sp-mixed polarization.

Another preferred variant is in 5 shown. Here is from 6 point-shaped light sources 5 (not shown), each light source 5 a linear polarizer 11 upstream in 45 ° sp mixing direction, in combination with a linear rotatable detection polarizer 12 , Several images with different detector polarizations are recorded and combined to form a resulting image.

The polarization between excitation polarizer 11 and detection polarizer 12 not set at 90 ° to each other, but at an angle equal to the corneal anterior reflex 6 is completely suppressed.

In all the above embodiments where orientations of polarizers 11 or analyzers 12 be varied to measure different corneal regions, is preferably a movie sequence with a camera 8th (in particular CMOS camera) recorded and subsequently in the evaluation 9 evaluated.

Instead of to change analyzing polarizers in their orientations, can also use fixed polarizers in combination with rotatable wave plates or polarization rotators are used. Thus, an arrangement can be realized with only one polarizer is used for excitation and detection.

As an alternative to rotatable polarizers, wave plates or rotators, sensor arrays can be used in which adjacent pixels with Such sensor arrays have been shown for another application in the lecture [6844A-43] at the Photonics West 2008 San Jose, CA, USA: A polarization measurement method for the quantification of retardation in optic nerve fiber layer , Yasufumi Fukuma, Topcon Medical Systems, Inc .; Yoshio Okazaki, Takashi Shioiri, Topcon Corp. (Japan); Yukio Iida, Komazawa Univ. (Japan); Hisao Kikuta, Osaka Prefecture Univ. (Japan); Kazuhiko Ohnuma, Chiba Univ. (Japan)).

The variation of polarization directions, also spatially resolved or radially symmetric, can also be effected by means of programmable liquid-crystal modulators, in particular low-scattered twisted-nematic LC arrays, as are known in the prior art (eg in US Pat T. Brixner et al "Femtosecond polarization pulse shaping"; Optics Letters Vol 26, 2001, p. 557 ).

When using a laser which generates even polarized light, such. As a Fabry-Perot laser, can be dispensed with the use of a polarizer. 6 shows a modification of the arrangement according to 2 , Here are polarizers 11 and analyzer 12 arranged in a common plane, what the in 4a to 4d simplified integration shown. In addition, this arrangement has diaphragms 13 for the light sources 5 on, which serve the correct adjustment of the distance between the measuring arrangement and the eye to be measured. For exact adjustment, the arrangement is brought to the eye until the reflex on the cornea is barely visible.

Alternatively, this adjustment can also be achieved by "focusing" the front reflexes 6 on the camera 8th will be realized.

Around Measuring errors due to patient movement are minimized of less than 50 ms preferred.

7 shows a preferred arrangement of the light sources 5 (seen in the axial direction of the eye) in the form of a regular hexagon. As sources of light preferably VCSEL sources are used (VCSEL = Vertical-Cavity Surface-Emitting Laser), z. ADNV-6340 from Avago Technologies, San Jose, CA, USA). With the help of other light sources 5 ' with other radial distances from the optical axis in addition radial changes in the radii of curvature, z. B. the asphericity of the area can be determined.

8th shows a third basic structure of an arrangement according to the invention. The light of a light source 5 , which is here preferably a semiconductor laser with a coherence length between 2 and 6 mm (eg HL 6711 Fa. Hitachi) is an imaging optics 14 collimates on the cornea 1 directed, from the front of the cornea 2 and the cornea back 3 Reflected light is produced by means of a semitransparent mirror 15 and a recording optics 16 to the camera 8th displayed. The coherence length of the light source is 5 chosen so that from the front of the cornea 2 and the cornea back 3 reflected light interferes with each other, indicating the camera 8th to an interferogram 17 leads. In this case, the interference fringes (also known as "Newtonian rings") can be used to obtain information about the deviation of the curvature between the anterior corneal surface using known methods of optical inspection technology 2 and cornea back 3 determine. The gain of the back-reflection signal due to the interference is particularly advantageous here: The resulting interference fringes have a significantly increased contrast with respect to the front-reflection signal of up to 1: 5, instead of the usual intensity contrast of about 1: 120.

The curvature of the front of the cornea 2 can also be in a known manner from the brightness distribution of the envelope of the light distribution of the interferogram 17 (spatial distribution of the amount of interference maxima).

In addition, a second light source can also be used 18 shorter coherence length (eg a super-luminescent diode SLD) by means of a coupler 19 with the beam path from the first light source 5 in order to be able to measure the front side curvature with suppression of the interference effects through the rear side, separately. Out of the camera 8th recorded brightness distribution 20 the reflection of the light of the second light source 18 on the front of the cornea 2 can be by conventional means the curvature of the front 2 determine. From this curvature and the also determined spatial deviation of the curvature between the front and back of the cornea 1 then can the radius of the cornea back 3 be easily calculated.

In another embodiment, of course, the curvature of the cornea front 2 also with one of the methods described above according to the prior art determine then together with the interferometrically determined curvature deviation on the radius of the back of the cornea 3 close.

The coherence length of 2 to 6 mm of the light source 5 in this embodiment, it has been determined to be greater than or equal to twice the optical thickness of the thickest corneas (2 x 650 μm xn _cornea with n _cornea = 1.38), but less than about twice the optical distance to other anterior chamber structures such as B. the crystalline lens (2 x 2.5 mm · n _aqueous with n _aqueous = 1.34). This is sure to prevent interference to lower-lying structures of the eye such. B. the eye lens distort the result.

The 9a and 9b show clippings from camera images of the arrangement 1 or 2 for each one light source 5 , The big spot of light 21 is the much brighter and larger front reflex 6 , the smaller spot of light 22 the backside reflex 7 , In 9a these are geometrically sufficiently separated and can therefore be evaluated well. In 9b lie the two reflexes 6 and 7 so close to each other that an evaluation is difficult. Here it has been shown that useful measurement results can also be obtained in this case by exploiting the interference. Is the coherence length of the light source 5 as in the arrangement after 8th in the range of 2 to 6 mm, front and rear reflex interfere with each other. It depends on the optical path length of the corneal thickness at the measuring point, whether more constructive (reinforcing) or destructive (extinguishing) interference results. In 9b Destructive interference is shown, ie the back reflex 7 deletes part of the light spot 21 of the front-side reflex 6 out. By slight variation of the wavelength of the light source 5 What can be realized in a VCSEL source, for example, by varying the current or temperature, can be successively set these two interference states and thus determine the position of the backside reflection despite spatial overlay with high accuracy. From the geometric sources of front and back reflex 6 . 7 can be described as the curvatures or radii of the front cornea 2 and cornea back 3 to calculate.

The invention is exemplary of a known pattern of discrete light sources 5 been described. Alternatively, it is of course also possible to use ring-shaped light sources such as the classic Placido disk or other point or distance patterns for the method according to the invention. It is advantageous if the patterns have sharply defined luminous surfaces to facilitate the separation of front and back reflexes.

By Use of several such ring light sources or annular Arranged point light sources can not just do a keratometry the cornea front and back but also determining higher deformation orders (Asphericity, etc.) to topographies of the anterior corneal and back.

1

cornea

2

Anterior corneal surface

3

Corneal back

4

crystal lens

5 and 5 '

light source

6

Anterior corneal reflex

7

Corneal reflex back

8th

camera (including optics)

9

evaluation

10

control unit

11

polarizer

12

analyzer

13

cover

14

imaging optics

15

The partly transparent mirror

16

recording optics

17

interferogram

18

SLD

19

coupler

20

brightness distribution

21

light spot Front Reflex

22

light spot Rear reflex

QUOTES INCLUDE IN THE DESCRIPTION

This list The documents listed by the applicant have been automated generated and is solely for better information recorded by the reader. The list is not part of the German Patent or utility model application. The DPMA takes over no liability for any errors or omissions.

Cited patent literature

• - US 6692126 [0003, 0034]
• US 6575573 [0003, 0034]
• US 7246905 [0003, 0007, 0034]
• - DD 251497 [0003, 0034]
• US 5886767 [0003]
• US 7154111 [0004]
• US 7133137 [0004]
• - DE 29913601 U1 [0005]
• - DE 29913602 U1 [0005]
• - DE 29913603 U1 [0005]
• - US 7264355 [0005]
• WO 2005/077256 [0007]
• US 7287885 [0039]

Cited non-patent literature

• DG Green et al in "Corneal thickness measured by interferometry", Journal of the Optical Society of America, Col. 65, February 1975, p. 119 [0004]
• - "Measurement of corneal thickness by low-coherence interferometry", Ch. K. Hitzenberger, Applied Optics Vol. 31, 1992, p. 6637 [0004]
• "Real-time pachymetry during photo-refractive keratectomy using optical low-coherence reflectometry", J. Biomediacal Optics Vol. 6, 2001, p. 412 [0004]
• J. Wang et al, "Noncontact Measurement of Central Corneal Epithelial and Flap Thickness of Old Laser In Situ Keratomileusis," Investigative Ophthalmology & Visual Science, Vol. 45, 2004, p. 1812 [0004]
• T. Brixner et al "Femtosecond polarization pulse shaping"; Optics Letters Vol 26, 2001, p. 557 [0056]

Claims (15)

Method for determining the inner radius of the cornea, at least one pattern of known geometry on the cornea is projected and that of the cornea back and preferred light reflected from the anterior corneal surface is analyzed. Method for determining the inner radius of the cornea according to claim 1, characterized in that the front of the cornea and the corneal back reflected light brought into interference and the resulting interference pattern is evaluated. Method for determining the inner radius of the cornea according to claim 1, characterized in that of the cornea back or cornea front reflected light geometrically evaluated becomes. Method for determining the inner radius of the cornea according to claim 3, characterized in that this geometric Evaluation by adjusting the intensities of the anterior corneal surface and back reflections by means of polarization optical means he follows. Method for determining the inner radius of the cornea according to claim 3, characterized in that the reflexes with a Image acquisition unit with a high dynamic range of preferably 120 be recorded to over 1000. Arrangement for determining the inner corneal radius in particular according to the method of claim 2, consisting of a Light source with a coherence length greater 2 mm, preferably in the range of 2 to 6 mm, a unit for production a pattern of known geometry, an imaging optic, which the test pattern on the cornea, a recording optics, a Image recording unit and an image evaluation unit. Arrangement for determining the inner corneal radius according to claim 6, characterized in that the image evaluation unit for analyzing the light reflected from the front of the cornea and for analysis of that by interference of the front and the back formed reflected light resulting interference pattern is. Arrangement for determining the inner corneal radius in particular according to the method of claim 3, consisting of at least a light source, a unit for generating a pattern known Geometry, at least one imaging optics, which the test pattern on the cornea, a recording optics, an image recording unit and an image evaluation unit. Arrangement for determining the inner corneal radius according to claim 8, characterized in that the image recording unit via a high dynamic range of preferably 120 to over 1000 features. Arrangement for determining the inner corneal radius according to claim 8, characterized in that in the imaging beam path at least one polarizer and in the recording beam path an analyzer is arranged. Arrangement for determining the inner corneal radius according to one of claims 6 to 10, characterized by that the pattern of known geometry consists of a structure of several Rings in the form of a Placido disk or discrete light sources consists. Arrangement for determining the inner corneal radius according to claim 11, characterized in that as discrete light sources VCSEL sources preferably with a wavelength between 650 and 1550 nm can be used. Arrangement for determining the inner corneal radius according to claim 12, characterized in that the VCSEL sources spectrally tunable are designed. Method for determining the inner radius of the cornea according to claim 2, characterized in that the interference pattern is evaluated for several phase angles, by variation the wavelength of the light sources during several measurements be achieved. Method for determining the inner radius of the cornea according to claim 3, characterized in that the wavelength the light sources used during a measurement quickly is varied to be undesirable in the geometric evaluation suppress interferometric signal components.