System for determining the topography of the cornea of ​​an eye

Abstract

System for determining the topography of the cornea of ​​an eye, comprising an element for generating rings similar to Placido disks and an illumination unit arranged in an illumination beam path, as well as an image acquisition unit arranged in a detection beam path and a control and evaluation unit, characterized in that the element for generating rings similar to Placido disks is a fresneled axicon (1) with ring-shaped structures of different radii, that an optical element for full-surface illumination of the fresneled axicon (1) with ring-shaped, plane waves and an optical element for separating the illumination and detection beam paths are arranged between the illumination unit and the fresneled axicon (1), that the imaging system (7.1) and an image sensor (7.2) are arranged in a control and evaluation unit.2) existing image acquisition unit (7) is designed for telecentric, distance-independent image acquisition and that the ring-shaped structures of different radii are arranged in the form of facets (1.3) with different facet angles on the front and / or back (1.1, 1.2) of the fresneled axicon (1), wherein the facet angles of the facets (1.3) of the fresneled axicon (1) are calculated such that the deflection of the light at small deflection angles is preferably based on the principle of light refraction and at large deflection angles on a combination of light reflection and light refraction.

Description

[0001] The present invention relates to a system for determining the surface shape of the cornea of ​​an eye by evaluating the mirror image of a spatially distributed pattern, in particular a ring pattern similar to a Placido disk.

[0002] The Placido disc is an illuminated disc that projects a familiar pattern of regularly spaced circles onto the eye. The diagnosis is then made by observing the reflection of these circles on the corneal surface, where they should also appear regularly spaced.

[0003] A symmetrical reflection of the concentric Placido rings should now be visible on the corneal surface. If, however, asymmetrical ring shapes are found, this indicates a deviation of the corneal surface from a reference surface. Irregularities of the corneal surface can be found, for example, in cases of astigmatism, but also in cases of mechanical or chemical injuries to the cornea.

[0004] Keratometry refers to the measurement of the shape and contour of the cornea. An ophthalmometer (also called a keratometer) is used to determine the central and peripheral radii of curvature of the cornea. These measured radii form the basis for fitting contact lenses, for example. A special form of keratometry is corneal topography. Here, specific techniques are used to measure and mathematically analyze the central and peripheral radii of curvature of the cornea.

[0005] Measuring the surface area of ​​the cornea of ​​the human eye proves difficult because the cornea is transparent and visible light is not backscattered to any significant extent.

[0006] The cornea is the outermost part of the eye and has a specific, curved shape. Topography is used to describe this geometric form of the corneal surface in its entirety. The shape of the anterior surface can vary, with two of the most important and widely known being the spherical (global) and the astigmatic (curved) cornea.

[0007] With a refractive power of over 40 diopters, the cornea is a crucial factor in the refraction of light entering the eye. The refractive power of the cornea depends primarily on the shape of its surface and, in particular, its curvature. Determining the shape of the corneal surface is of utmost importance in the following applications: • Cataract surgery, in conjunction with astigmatism reduction, • Contact lens fitting, • Detection of keratoconus, • corneal determination of astigmatism, • refractive surgery and • Fitting of suitable glasses.

[0008] In surgical applications, determining the surface shape of the cornea of ​​an eye is important both before and after the surgical procedure, as the surface shape is suitable for detecting anomalous or abnormal corneal shapes.

[0009] Methods for measuring corneal surface shape using so-called keratometers or keratographs have long been known in the art. The concentric rings of the Placido disc, projected onto the cornea, are reflected by the corneal tear film and captured and analyzed by a camera. Depending on the corneal curvature, the reflected ring pattern detected by the camera is distorted. To determine the curvature from these reflection signals, the distortions of the rings must be compared with a known shape, usually a sphere with a radius of 7.8 mm. Such a solution is described, for example, in US 4,685,140 A.

[0010] The Placido disks used by topographers to generate concentric rings do not necessarily have to be flat disks. While such flat Placido disks are well known in the prior art and described, for example, in US 5,110,200 A and US 5,194,882 A, funnel-shaped (US 5,684,562 A, US 6,116,738 A) or spherically convex (US 5,864,383 A) Placido disks are more common.

[0011] Documents US 6,575,573 B2 and US 6,692,126 B1 describe solutions for ophthalmometers (also called keratometers) supplemented by a slit illumination unit. While the imaging of placidoring systems is intended for measuring the surface curvature of the cornea, the slit illumination unit generates cross-sectional images of the eye from which the corneal thickness can be determined. The result of this combination is a corneal thickness profile.

[0012] This method has the advantage that only a single image is required to measure the corneal surface shape, and this image contains a large number of data points at high spatial resolution. Because only a single image is needed, eye movements do not affect the accuracy of the measurement. A primary disadvantage of such solutions is that the images captured by the camera contain a superimposed image of the eye, particularly its iris, and the reflection of the Placidor rings projected onto the tear film in front of the cornea.

[0013] For an accurate determination of the eye's surface topography, the evaluation algorithm must reliably detect the Placido rings. However, a problem can arise here: the superimposed iris structure can interfere with the detection of the Placido rings, as the pupil margin may be mistaken for the rings.

[0014] Commercially available corneal topography systems project real Placido rings onto the cornea at a short distance in front of the eye, from where they are reflected and captured by a camera. Corneal reconstruction is based on the angular analysis of the angles of incidence and reflection of the projected Placido rings as reflected by the cornea. The deviation of the ring position on the cornea relative to the ring position of a known reference test object serves as the basis for corneal reconstruction. A second disadvantage of such solutions is that the accuracy of the measurement is highly dependent on the angular relationships and thus on the measurement distance.

[0015] Various methods are used to determine and verify the correct measuring distance. For example, the measurement can be triggered automatically when the correct working distance is reached. This can be achieved, on the one hand, by correcting the incorrect distance before each measurement, using light barriers, contacts, or additional measuring systems to determine and, if necessary, correct the distance or position.

[0016] Examples of such solutions are US 6,048,065 A and US 6,070,981 A. The solutions described therein are topographs based on a Placido disk. To verify the correct measuring distance, both solutions employ a point light source whose light is projected onto the cornea, reflected, and imaged as a dot pattern onto a CCD camera. The position of this dot pattern within the image area indicates the distance between the Placido disk and the eye. For precise positioning, the Placido disk is moved until the distance is optimized. Only then is the measurement initiated.

[0017] In the solutions described in EP 0 843 529 B1 and EP 1 138 254 A1, the correct measuring distance is checked and corrected using two additional cameras, one positioned to the right and one to the left of the head. Both the two laterally positioned cameras and the main camera, located on the optical axis, are rigidly aligned relative to each other for measuring the cornea. Based on the images captured by the laterally positioned cameras, the entire system is positioned relative to the eye being measured, or rather its cornea, to achieve an optimal measuring position.

[0018] On the other hand, measurements taken at suboptimal distances can also be corrected by applying specific correction algorithms for cases where, for example, image analysis reveals suboptimal measurement distances. These algorithms can then correct the erroneous measurement results.

[0019] The world's most widely used corneal topography system from Carl Zeiss Meditec is called ATLAS™ and features such correction algorithms. Thanks to its powerful and user-friendly software platform, the ATLAS™ system delivers precise, reliable, and reproducible results for virtually any operator in every examination.

[0020] The patented Cone-of-Focus™ alignment system of the ATLAS™ ensures that the cornea being measured is correctly aligned with the measuring system. This is achieved through triangulation using Cone-of-Focus™ in conjunction with the patented Placido disc.

[0021] Although this procedure provides precise, reliable, and reproducible results for the majority of eyes examined—that is, for eyes with "normal" corneal curvature—problems can sometimes arise with "abnormal" corneal curvature. Such an "abnormally" curved cornea can, for example, be caused by a pre-existing condition like keratoconus.

[0022] To determine the precise angular relationships, the distance of the device from the point on the eye being examined where the radiation from a Placidor ring is reflected must be known. Therefore, not only must the distance of the eye from the measuring device on the optical axis be known, but also the exact shape of the cornea, which is yet to be measured. Interactive algorithms can be used to minimize the influence of this problem on the measurement results. However, these algorithms combine the measurement uncertainties of all rings through error propagation, making the measurement results inaccurate depending on the data quality. In patients with healthy corneal shapes, this problem is manageable due to the very high measurement quality; however, very large measurement deviations can occur in pathologically deformed corneas.

[0023] The precise, high-accuracy alignment, accurate to less than 1 micrometer, creates the foundation for highly accurate measurements in every examination. The SmartCapture™ image analysis system analyzes up to 15 digital images per second during alignment and automatically selects the image with the best quality.

[0024] The idea of ​​precisely examining and measuring the cornea of ​​the human eye is very old. The first classic keratometer was developed by H. von Helmholtz as early as 1856. The measurement method it enabled was the same as the one still used today. However, Helmholtz's keratometer was very unwieldy.

[0025] It is all the more surprising that Littmann only succeeded in developing a completely distance-independent keratometer in 1950. With distance-independent keratometers, the image sharpness settings are not affected by the observer's accommodation or refractive errors, thus eliminating the major shortcomings of distance-dependent instruments.

[0026] In the Littmann keratometer, virtual test symbols are projected onto the cornea via a lens, projected to infinity. This means that the distance dependence of the measurement is solely on the depth of field of the camera, which serves as the optical observation system, and not on the distance of the test symbols. Distance independence in image acquisition is achieved through a telecentric beam path. This significantly reduces the sensitivity of the measurement accuracy to the distance between the eye and the device, resulting in highly repeatable measurement results. Furthermore, the number of measurement and reading errors has been reduced, and the measurement process has been accelerated.

[0027] However, the small number of measurement points in Littmann keratometers proves to be a disadvantage. While Littmann keratometers only generate 2 to 8 measurement points, the number of measurement points in topographs based on Placido disks can reach several thousand.

[0028] A completely distance-independent technical solution is provided by keratometers, in which, for example, six individual collimated light sources illuminate the cornea at specific angles. The key advantage of such methods lies in the highly accurate, quantitative measurement of corneal curvature. The IOLMaster® from Carl Zeiss Meditec AG, based on a short-coherence method, is an optical measuring device based on this principle.

[0029] A disadvantage of this method is that the cornea is measured at only a few points, which can lead to measurement errors, especially in corneas that exhibit surface deformations in addition to curvature and astigmatism. For such exceptional cases, a large number of measurement points, similar to a Placido disk, would be advantageous. However, since a separate source with imaging optics is required for each measurement point, the technical effort increases accordingly.

[0030] German patent DE 1 572 779 A describes a method in which illumination is achieved using an aperture whose several narrow, ring-shaped, concentric openings are imaged onto the eye being measured via an optical system. Detection, as in all previously described methods, is performed through the center of the ring system using telecentrically corrected optics along the optical axis. The distance-independent illumination is achieved by illuminating the eye with several "ring-shaped plane" waves. The optics that image the ring apertures onto the eye are thus positioned at the same distance from both the ring aperture and the cornea of ​​the eye, with the principal plane corresponding to its resulting focal length.A disadvantage of this method is the relatively high technical complexity of the imaging optics, as several lenses are required to achieve the necessary high measurement accuracy and to compensate for any spherical errors. Another disadvantage is the low light efficiency, resulting from the relatively narrow ring aperture widths required.

[0031] The present invention aims to develop a system for determining the topography of the cornea of ​​an eye, which allows for largely distance-independent measurements and generates a large number of measurement points. For illumination, a technical design is to be used that exhibits higher luminous efficiency and can be manufactured much more simply and therefore more cost-effectively.

[0032] The underlying problem is solved by the system for determining the topography of the cornea of ​​an eye, consisting of an element for generating rings similar to Placido disks and an illumination unit arranged in an illumination beam path, as well as an image acquisition unit arranged in a detection beam path and a control and evaluation unit, in that the Placido disk-like element is designed as a fresneled axicon with ring-shaped structures of different radii, that an optical element for full-surface illumination of the fresneled axicon with plane waves and an optical element for separating the illumination and detection beam paths are arranged between the illumination unit and the fresneled axicon, and that the image acquisition unit, consisting of an imaging system and an image sensor, is designed for telecentric, distance-independent image acquisition.

[0033] According to the invention, the problem is solved by the features of the independent claims. Preferred embodiments and configurations are the subject of the dependent claims.

[0034] The proposed system is suitable for determining the topography of reflecting bodies. In particular, however, it serves to determine the topography of the cornea of ​​an eye.

[0035] The invention is described in more detail below with reference to exemplary embodiments. To this end, we show... Fig. 1: The schematic representation of the fresneled axicon according to the invention, with facets on the front and back, Fig. 2: The schematic representation of the fresneled axicon according to the invention with an additional mirror surface, Fig. 3: The schematic representation of the system according to the invention with fresneled axicon, collimator lens and beam splitter, Fig. 4: The schematic representation of the system according to the invention with fresneled axicon and a beam splitter, Fig. 5: The schematic representation of the system according to the invention in which the detection beam path is deflected, Fig. 6: The schematic representation of the system according to the invention with fresneled axicon and a collimator lens, Fig. 7: The schematic representation of the system according to the invention, whose fresneled axicon has an additional collimating and beam-path separating effect, Fig. 8: the schematic representation of the system according to the invention, in whose fresneled axicon the imaging optics of the image acquisition are integrated, Fig. 9: a preferred embodiment of the fresneled axicon according to the invention, the front of which has facets and the back of which has an aspherical shape, Fig. 10: a further preferred embodiment of the fresneled axicon according to the invention, which, in addition to facets on the front and an aspherical shape on the back, has an outer facet common to all zones and Fig. 11: the preferred beam path for illumination and detection for the design according to Fig. 10.

[0036] In all illustrations, the front surface of the element facing the cornea is defined as the front side. Even if the cornea is not shown, the front side is always depicted on the left.

[0037] The system according to the invention for determining the topography of the cornea of ​​an eye consists of an element for generating rings similar to Placido disks and an illumination unit, both arranged in an illumination beam path, as well as an image acquisition unit arranged in a detection beam path and a control and evaluation unit. The element for generating rings similar to Placido disks is a fresneled axicon with ring-shaped structures of varying radii. Furthermore, an optical element for the full-surface illumination of the fresneled axicon with plane waves and an optical element for separating the illumination and detection beam paths are arranged between the illumination unit and the fresneled axicon. In addition, the image acquisition unit, consisting of an imaging system and an image sensor, is designed for telecentric, distance-independent image acquisition.

[0038] A first group of advantageous embodiments concerns the fresneled axicon used as an element for generating rings. Here, the ring-shaped structures of different radii can be arranged as facets on the front and / or back side of the fresneled axicon.

[0039] The Fig. Figure 1 shows the principle representation of the fresneled axicon according to the invention, with facets on the front and back.

[0040] Both the front 1.1 and the back 1.2 of the fresneled axicon 1 feature facets 1.3 that direct the light from the illumination source onto the cornea at different angles. Furthermore, the fresneled axicon 1 has a central zone 1.4, designed as a flat surface or recess, which is intended for telecentric, distance-independent image acquisition.

[0041] By arranging facets on the front and back surfaces, the number of ring-shaped structures, and thus the number of measurement points, could be significantly increased. These ring-shaped structures, with their varying radii, exhibit different facet angles to redirect the light at different angles towards the cornea of ​​the eye.

[0042] Although the deflection of light can in principle be based on the principles of refraction, diffraction or reflection, the principle of diffraction is usually avoided.

[0043] This is because light diffraction at rotationally symmetric grating structures has a significant drawback. The deflection angle due to diffraction scales with the wavelength of the radiation used. Therefore, in such a case, the spectral bandwidth of the light source must be limited to below 1 nm to obtain accurate measurements, which can only be achieved technically with considerable effort using narrowband light sources such as lasers or by means of additional filter elements.

[0044] Therefore, the following must be considered when estimating the necessary accuracy. Determining the refractive power of the cornea by measuring its curvature with a desired accuracy of < 0.1 diopters corresponds to approximately 1 / 500 for a total corneal refractive power of 40–50 diopters. This means that the light source must have a bandwidth of less than 2 nm, preferably less than 1 nm, at a wavelength of, for example, 800 nm. Furthermore, the mean wavelength must not vary by more than approximately ±0.5 nm. To achieve these conditions for the light source, typical semiconductor laser sources offering such a narrow bandwidth must be temperature-controlled to approximately 1 K, which would entail additional technical effort.

[0045] Therefore, the facet angles of the fresneled axicon are calculated such that the deflection of light at small deflection angles is preferably based on the principle of refraction and at large deflection angles on the principle of reflection, partly also in combination with refraction.

[0046] In a particularly advantageous embodiment, the facet angles of the fresneled axicon are calculated such that the deflection of the light at large deflection angles is based on the principle of light reflection, for which the fresneled axicon has an additional mirror surface.

[0047] The Fig. Figure 2 shows the principle representation of the fresneled axicon according to the invention with an additional mirror surface (cone).

[0048] In addition to the facets 1.3 present only on the front surface 1.1, the freted axicon 1 has an outer facet 1.5 serving as a common mirror surface. For telecentric, distance-independent image acquisition, the central zone 1.4 is again designed as a flat surface or recess.

[0049] The light from the outer zones undergoes the main part of its deflection through total internal reflection at the mirror surface common to all outer zones, in the form of an outer facet. The final deflection is then achieved by the facets on the front of the fresneled axicon. Such an element allows for more favorable conditions for the outer zones with a large angle of incidence.

[0050] A second group of advantageous embodiments relates to the illumination unit. This preferably consists of at least one light source, in particular an LED, a laser, or the output of an optical fiber. Since the setup is very light-efficient, other types of non-point sources, such as halogen lamps and flash lamps, can also be used. However, these must be limited to a spectral bandwidth below 50 nm by an additional filter in order to obtain accurate measurement results, as the angle of refraction also depends on the refractive index of the material due to its dispersion.

[0051] The light from the light source, which for example has a luminous area of ​​approximately 1mm, can 2 additionally, it can be occupied by means of a circular aperture with a diameter of 100 - 1000µm.

[0052] In an advantageous embodiment, the illuminating surface of the light source is variable. This can be achieved, for example, by a pre-arranged aperture with a variable diameter or by several light sources with differently sized illuminating surfaces that are used alternately.

[0053] This has the advantage that the width of the rings displayed and projected onto the eye can be changed. For example, in the case of a severely deformed cornea, it is necessary to use very narrow rings that do not touch, thus enabling trouble-free evaluation. Wider rings could touch or even overlap in this case, making evaluation difficult or even impossible.

[0054] Between the illumination unit and the fresneled axicon, an optical element for the full-surface illumination of the fresneled axicon with plane waves and an optical element for separating the illumination and detection beam paths are arranged. The two optical elements are designed and arranged in such a way as to prevent illumination of the central zone of the fresneled axicon, which is intended for telecentric image acquisition.

[0055] According to an advantageous embodiment, the optical element for the full-surface illumination of the fresneled axicon with plane waves is a collimator lens. This is shown in the Fig. 3 the schematic representation of the system according to the invention with collimator lens and beam splitter.

[0056] The light emitted by an LED 2 is collimated by a collimating lens 3 and deflected by a beam splitter in the form of a partially mirrored, plane-parallel plate 4 towards the fresneled axicon 1. Optionally, an aperture 2.1 (shown with a dashed line) can be positioned in front of the LED 2 to limit the illumination field. The partial mirroring of the plane-parallel plate 4 can be designed such that its center is not mirrored, thus preventing illumination of the central zone 1.4 of the fresneled axicon 1. If, on the other hand, the central zone is partially mirrored, a reflection point on the apex of the cornea can be observed in the camera image next to the ring system, which can be advantageously useful for evaluation. However, this has the disadvantage that the light of the detection beam path is attenuated by the partially mirrored layer, thus reducing the light efficiency.For this reason, the partial mirroring of the central areas of the deflecting element can be adapted to the desired measurement characteristics. The light from the LED 2 is directed at different angles onto the cornea 5.1 of the eye 5 via the ring-shaped facets 1.3 of the fresneled axicon 1, which have different angles. The ring structure projected onto the cornea 5.1 of the eye 5 is reflected along the telecentric beam path 6 through the central zone 1.4 of the fresneled axicon 1, which is designed as a flat surface or recess. The image is then captured by the image acquisition unit 7 and forwarded to the control and evaluation unit (not shown) for processing. The image acquisition unit 7 has an imaging optic 7.1 and an image sensor 7.2 for this purpose.

[0057] In a particularly advantageous embodiment, the collimator lens, which is usually used as an optical element for the full-surface illumination of the fresneled axicon, can be omitted if the back side of the fresneled axicon is designed accordingly. This is shown in the Fig. 4 the schematic representation of a second embodiment of the system according to the invention with only one beam splitter.

[0058] The light emitted by an LED 2 is deflected by a beam splitter in the form of a partially mirrored, plane-parallel plate 4 towards the fresneled axicon 1. The rear side 1.2 of the fresneled axicon 1, designed as a collimator lens 3', aligns the light for full-surface illumination, thus eliminating the need for a separate optical element. The partial mirroring of the plane-parallel plate 4 can be configured such that its center is not mirrored, thereby preventing illumination of the central zone 1.4 of the fresneled axicon 1. Both the imaging of the light from the LED 2 onto the cornea 5.1 of the eye 5 at various angles and the image acquisition are carried out in the manner described above.

[0059] Planar plates or prisms, which may have partially mirrored and / or dichroic surfaces, are preferably used as optical elements to separate the illumination and detection beam paths.

[0060] For the functionality, it is irrelevant whether the illumination or the detection beam path is deflected via the optical element. However, it is more advantageous to deflect the detection beam path, as the dimensions of the required optical element, in the form of a flat plate or a prism, can be smaller. This is shown in the... Fig. 5 the schematic representation of the system according to the invention in which the detection beam path is deflected.

[0061] According to the Fig. The light emitted by an LED 2 is collimated by a collimating lens 3 and directed towards the fresneled axicon 1. The light from the LED 2 is directed at different angles onto the cornea 5.1 of the eye 5 via the differently angled facets 1.3 of the fresneled axicon 1. The ring structure projected onto the cornea 5.1 of the eye 5 is reflected along the telecentric beam path 6 through the central zone 1.4 of the fresneled axicon 1, which is designed as a planar surface or recess. It is then deflected by a plane-parallel plate 8 and imaged onto the image acquisition unit 7. By positioning the plane-parallel plate 8 in front of the fresneled axicon 1, illumination of its central zone 1.4 is avoided.

[0062] Even with this design, a variant is conceivable in which the number of optical elements can be reduced. For example, the separate optical element for separating the illumination and detection beam paths can be omitted if the back of the fresneled axicon is designed accordingly. This is shown in the Fig. 6 The schematic representation of the system according to the invention with only a collimator lens.

[0063] The light emitted by an LED 2 and collimated by a collimator lens 3 is directed towards the fresneled axicon 1. The collimator lens 3 may have a partial reflective coating in its center, which prevents the central zone 1.4 of the fresneled axicon 1 from being illuminated. The rear side 1.2 of the fresneled axicon 1, designed as a prism 4', separates the illumination and detection beam paths, thus eliminating the need for a separate optical element. Both the imaging of the light from the LED 2 onto the cornea 5.1 of the eye 5 at various angles and the image acquisition are performed in the manner described above.

[0064] In a particularly advantageous embodiment, separate optical elements for both separating the illumination and detection beam paths and for full-surface illumination can be dispensed with if the back of the fresneled axicon is designed accordingly. This is shown in the Fig. 7 The schematic representation of the system according to the invention with a fresneled axicon, the back side of which is designed in such a way that it has a collimating and beam path separating effect.

[0065] The light emitted by an LED 2 is directed towards the fresneled axicon 1. The rear side 1.2 of the fresneled axicon 1, designed as a wedge lens 9', performs both the separation of the illumination and detection beam paths and the full-surface illumination of the fresneled axicon 1, thus eliminating the need for separate optical elements for this purpose.

[0066] The rear surface 1.2 of the fresneled axicon 1, designed as a wedge lens, has a curved, ring-shaped prismatic form and a corresponding mirrored or dichroic surface to prevent illumination of the central zone 1.4 of the fresneled axicon 1. Both the imaging of the light from the LED 2 onto the cornea 5.1 of the eye 5 at different angles and the image acquisition are carried out in the known manner.

[0067] In another advantageous embodiment, the facets on the front of the fresneled axicon are designed to have an aspherical effect for correcting aberrations. This makes it possible to correct aberrations such as chromatic aberration, distortion, and the like.

[0068] A third group of advantageous designs concerns the image acquisition unit, which typically consists of an imaging optic and an image sensor. For its telecentric image acquisition, the central zone of the fresneled axicon is designed as a hole or a flat surface.

[0069] However, it can also be advantageous if the central zone of the fresneled axicon has an optical function. For example, the central zone can function as a filter or as an imaging optic. This is shown in the Fig. 8 the schematic representation of the system according to the invention in whose fresneled axicon the imaging optics of the image acquisition are integrated.

[0070] According to the with Fig. In the solution described in Figure 7, the light emitted by an LED 2 is directed onto the rear side 1.2 of the fresneled axicon 1, which is designed as a wedge lens 9'. The light from the LED 2 is imaged onto the cornea 5.1 of the eye 5 at various angles and reflected from there along the telecentric beam path 6 onto the image sensor 7.2 of the image acquisition unit 7. Since the central zone 1.4 of the fresneled axicon 1 is designed as the imaging optic 7.1, a separate imaging optic is not required.

[0071] The image of the ring structure reflected by the cornea 5.1 of the eye 5, captured by the image sensor 7.2, is forwarded to the (not shown) control and evaluation unit for evaluation, as already described.

[0072] One particularly advantageous design variant is shown by the Fig. 9. The fresneled axicon 1 shown here is designed such that its front surface 1.1 has facets 1.3 and its back surface 1.2 has an aspherical shape, and the fresneled axicon 1 also has a curved or conical overall shape. The collimation of the wave emitted by the light source is achieved by an aspheric surface on the back surface 1.2, and the deflection of the annular plane waves is achieved by the facets 1.3 of the fresneled axicon 1 formed on its front surface 1.1. However, the light reaching the outer 11 zones is not achieved by refraction at the fresneled axicon 1, but rather by a combination of total internal reflection and refraction at two facets 1.3 of the fresneled axicon 1 each.

[0073] Another, particularly advantageous design variant is shown by the Fig. 10. The fresneled axicon 1 shown here is a modification of the structure in Fig. 9 and is characterized in that the light from the outer 11 zones undergoes the main part of its deflection by total internal reflection at the outer facet 1.5 common to all outer zones, and the final beam deflection is achieved by a facet 1.3 on the front of the fresneled axicon 1. This shape of the fresneled axicon 1 has the advantage of providing more favorable conditions for the outer zones with a large angle of incidence onto the cornea.

[0074] The Fig. Figure 11 shows the preferred beam path for illumination and detection for the design according to Fig. 10. The light emitted by an LED 2 is directed towards the fresneled axicon 1. The rear surface 1.2, designed as a collimating lens 3', parallelizes the light for full-surface illumination of the fresneled axicon 1, thus eliminating the need for separate optical elements for this purpose. The light beams 10 and 11 of the outer zones undergo total internal reflection at the common outer facet 1.5 and are then deflected by a facet 1.3 on the front of the fresneled axicon 1 to image the cornea 5.1. In contrast, the light beams 12 and 13 of the inner zones are deflected only by a single facet 1.3 each. The ring structure projected onto the cornea 5.1 of the eye 5 is reflected as a bundle of rays 16 through the central zone 1.4 of the fresneled axicon 1, which is designed as a planar surface or recess, by a first plane-parallel plate 8.The beam is deflected via an imaging optic 7.1 towards the image sensor 7.2. A second plane-parallel plate 8.2 is located in the imaging optic 7.1. The double deflection of the detection beam path results in a very compact arrangement.

[0075] In a further variation, the fresneled axicon can be designed in such a way that both the collimating effect for the light source and the deflecting effect for beam deflection towards the cornea are realized on only one side, for example the front of the fresneled axicon.

[0076] The solution according to the invention provides a system for determining the surface shape of the cornea of ​​an eye, which is based on a Placido disk and an image acquisition unit with telecentric image acquisition, has a simple and inexpensive design, higher light efficiency and yet enables largely distance-independent measurements of corneal topography.

[0077] The use of a placido disk-like element generates a large number of measurement points, which inevitably leads to an increase in the accuracy of determining the topography of the cornea.

[0078] The proposed system for determining the surface shape of the cornea of ​​an eye thus combines the advantages of a Placido disk-based topograph (multiple measurement points) with those of a Littmann keratometer (distance-dependent measurements). This is achieved by designing the Placido disk as a fresneled axicon with ring-shaped structures of varying radii that project the light from a single light source at different angles towards the cornea of ​​the eye.

[0079] The particular advantage of the system according to the invention for determining the topography of the cornea of ​​an eye is primarily that the proposed formation of the ring structure is achieved without any absorption, i.e., solely by light shaping and guidance, requiring only a single light source.

[0080] Although the proposed system is specifically intended for determining the topography of the cornea of ​​an eye, it can be used to determine the topography of all reflecting bodies.

[0081] A topograph based on a Placido disc provides a signal with the typical rings for all distances between the device and the eye. However, measurement errors occur due to the distance dependence, which scale with the distance to the optimal focus / adjustment point.

[0082] In the distance-independent system presented here, similar to Littmann keratometers, the effect is observed that larger deviations from the optimal adjustment point result in shadowing effects, and no rings are observed in the camera image. As soon as one approaches the adjustment point again, the rings become visible, and very accurate measurements can be calculated from this, independent of the distance.

[0083] The area in which no shadowing effects occur depends essentially on the width of the ring-shaped, "planar" beams and the mean angle of the corresponding zone. It is therefore preferable to optimize the width of the zones independently of each other to achieve the largest possible free adjustment range, in order to enable robust measurements even with unsteady eyes. Therefore, the outer zones are, for example, designed according to the Fig. 9 and Fig. The outer rings are significantly wider than the inner ones. For certain measuring instruments, it may be necessary to allow for a much more robust setting when measuring a reduced number of rings. Therefore, it can be advantageous to implement an element with only a few, but significantly larger, widths. Elements with only 3 rings / zones are conceivable, for example.

[0084] Furthermore, each of the depicted structures can be scaled so that a certain insensitivity to adjustment can be achieved for a specific number of zones.

[0085] Furthermore, it is possible to set the adjustment range of the different zones to different values ​​and to use the number and number of unshaded zones in the camera image to implement automatic or semi-automatic positioning of the device in front of the eye. For example, the maximum and minimum zone number visible in the camera image can be used as a control signal for an axial actuator.

Claims

[1] System for determining the topography of the cornea of ​​an eye, consisting of an element for generating rings similar to Placido discs and an illumination unit arranged in an illumination beam path, as well as an image acquisition unit arranged in a detection beam path and a control and evaluation unit, characterized by, that the element for generating rings similar to Placido disks is a fresneled axicon (1) with ring-shaped structures of different radii, that an optical element for full-surface illumination of the fresneled axicon (1) with ring-shaped, plane waves and an optical element for separating the illumination and detection beam paths are arranged between the illumination unit and the fresneled axicon (1), that the image acquisition unit (7) consisting of an imaging system (7.1) and an image sensor (7.2) is designed for telecentric, distance-independent image acquisition, and that the ring-shaped structures of different radii are arranged in the form of facets (1.3) with different facet angles on the front and / or back (1.1, 1.2) of the fresneled axicon (1), wherein the facet angles of the facets (1.3) of the fresneled axicon (1) are calculated such that the deflection of the light at small deflection angles is preferably based on the principle of light refraction and at large deflection angles on a combination of light reflection and light refraction. [2] System according to claim 1, characterized by , that the facet angles of the facets (1.3) of the fresneled axicon are calculated such that the deflection of the light at large deflection angles is based on the principle of light reflection on a surface common to all zones, for which the fresneled axicon (1) has an additional mirror surface (1.5). [3] System according to claims 1 and 2, characterized by , that the lighting unit has at least one light source (2) to which a variable diameter aperture (2.1) is arranged. [4] System according to at least one of the preceding claims, characterized by, that the lighting unit has several light sources (2) with differently sized illuminating surfaces which can be used alternately. [5] System according to at least one of the preceding claims, characterized by , that the optical element for the full-surface illumination of the fresneled axicon (1) with ring-shaped, plane waves is a collimator lens (3). [6] System according to at least one of the preceding claims, characterized by , that the back of the fresneled axicon (1) is designed such that the collimator lens (3) can be omitted. [7] System according to at least one of the aforementioned claims, characterized by , that the ratio of the luminous area of ​​the light source (2) to the focal length of the collimator lens (3) has a value between 1:20 and 1:1000, in particular 1:100 to 1:

250. [8] System according to at least one of the aforementioned claims, characterized by, that the optical element for separating the illumination and detection beam paths is a plane plate (4) or a prism (4) with partially mirrored and / or dichroic surfaces. [9] System according to at least one of the preceding claims, characterized by , that the back side of the fresneled axicon (1) is designed in such a way that the optical element for separating the illumination and detection beam paths can be omitted. [10] System according to at least one of the preceding claims, characterized by , that the back side of the fresneled axicon (1) is designed in such a way that both the optical element for separating the illumination and detection beam paths and the collimator lens (3) can be omitted. [11] System according to at least one of the preceding claims, characterized by, that the facets on the front face (1.1) of the fresneled axicon (1) are designed to have an aspheric effect for correcting aberrations and / or collimating the illumination light. [12] System according to at least one of the preceding claims, characterized by , that the central zone (1.4) of the fresneled axicon (1) intended for telecentric image acquisition is designed as a hole or planar surface. [13] System according to at least one of the preceding claims, characterized by , that the central zone (1.4) of the fresneled axicon (1) intended for telecentric image acquisition is designed in such a way that the imaging system (7.1) of the image acquisition unit (7) can be omitted. [14] System according to at least one of the preceding claims, characterized by, that the width of the bundles incident on the cornea (5.1) is adjusted so that the largest possible free adjustment range of the eye (5) in front of the system can be achieved. [15] System according to at least one of the preceding claims, characterized by , that the free adjustment range for the zones is designed to be so different that the shading effects of the zones are detected, evaluated and used as a control signal for automatic or semi-automatic positioning of the system in front of the eye. [16] System according to at least one of the preceding claims, characterized by , that the optically effective surfaces of the fresneled axicon (1) have no scattering effect.

Images