METHOD AND DEVICE FOR MEASURING AN OPTICAL LENS FOR INDIVIDUAL WEARING SITUATIONS BY A USER

Abstract

An apparatus for measuring a cornea of a subject contains an image capturing device configured to capture image data of an iris of the subject from a plurality of viewpoints by imaging beam paths which pass through the cornea and a computing unit for providing a mathematical model of an anterior eye section of the subject including a mathematical model of the cornea and the iris. The model further identifies and registers image features of the iris, which are present in a plurality of images of the image data; determines deviations between actual positions of the image features of the iris in the images captured from the plurality of viewpoints and expected positions of the image features of the iris in the images captured from the plurality of viewpoints in consideration of the mathematical model of the cornea and the position of the iris.

Claims

1. An apparatus for measuring a cornea of a subject, the apparatus comprising: an image capturing device configured to capture image data of an iris of the subject from a plurality of viewpoints by imaging beam paths which pass through the cornea; and a computing unit configured to: provide a mathematical model of an anterior eye section of the subject including a mathematical model of the cornea and the iris; identify and register image features of the iris, which are present in a plurality of images of the image data; determine deviations between actual positions of the image features of the iris in the images captured from the plurality of viewpoints and expected positions of the image features of the iris in the images captured from the plurality of viewpoints in consideration of the mathematical model of the cornea and the position of the iris; adapt parameters of the mathematical model of the cornea so as to minimize the deviations; and determine a metric of the cornea from the adapted mathematical model of the cornea.

2. The apparatus as claimed in claim 1, wherein the computing unit is configured to evaluate a refractive power or an astigmatism from the adapted mathematical model of the cornea.

3. The apparatus as claimed in claim 1, wherein the mathematical model comprises a relative position of the iris with respect to the cornea.

4. The apparatus as claimed in claim 1, wherein the apparatus is configured to calculate the cornea situated between the iris and the image capturing device without prior knowledge about how the iris looks.

5. The apparatus as claimed in claim 1, wherein the apparatus is configured to perform a correlation of previously unknown image features in images of the iris recorded from different viewpoints.

6. The apparatus as claimed in claim 1, wherein the computing unit is configured to calculate the shape of the cornea based on a system of equations from imaging beam paths which are captured at the respective known positions.

7. The apparatus as claimed in claim 1, wherein the image capturing device comprises: a first camera configured to capture first image data from a first viewpoint; and a second camera configured to capture second image data from a second viewpoint, wherein the computing unit is configured to determine the three-dimensional shape of the cornea on the basis of the first image data and the second image data.

8. The apparatus as claimed in claim 1, wherein the computing unit is configured to iteratively determine the three-dimensional shape of the cornea with an integration method.

9. The apparatus as claimed in claim 1, wherein the computing unit is configured to determine the three-dimensional shape of the cornea on a basis of back tracing the respective imaging beam paths entering the image capturing device.

10. The apparatus as claimed in claim 1, wherein the computing unit is configured to determine the three-dimensional shape of the cornea by dividing at least one of a front surface or a back surface of the cornea into surface elements and to determine an alignment of the surface elements.

11. The apparatus as claimed in claim 10, wherein the computing unit is configured to determine a three-dimensional shape of the front surface and the back surface of the cornea on the basis of the alignment of the surface elements.

12. The apparatus as claimed in claim 1, wherein the computing unit is configured to determine the three-dimensional shape of the cornea taking account of a boundary condition that a front surface or a back surface of the cornea has a parameterizable area.

13. The apparatus as claimed in claim 12, wherein the parameterizable area includes a sphere, a section of the sphere, a torus, or a section of the torus.

14. The apparatus as claimed in claim 1, wherein the computing unit is further configured to determine the three-dimensional shape of the cornea taking account of one or more known contact points of the cornea.

15. The apparatus as claimed in claim 1, wherein the computing unit is configured to determine a spatial refractive index distribution of the cornea to be measured.

16. A method for measuring the cornea of a subject, the method comprising the steps of: capturing image data of an iris of the subject from a plurality of viewpoints by imaging beam paths which pass through the cornea; providing a mathematical model of an anterior eye section of the subject including a mathematical model of the cornea and the iris; identifying and registering image features of the iris, which are present in a plurality of images of the image data; determining deviations between actual positions of the image features of the iris in the images captured from the plurality of viewpoints and expected positions of the image features of the iris in the images captured from the plurality of viewpoints in consideration of the mathematical model of the cornea and the position of the iris; adapting parameters of the mathematical model of the cornea so as to minimize the deviations; and determining a metric of the cornea from the adapted mathematical model of the cornea.

17. The method of claim 16, further comprising a preceding step of calibrating the camera system for capturing the image date of the iris from the plurality of viewpoints.

18. A computer program product being stored on a non-transitory storage medium and comprising instructions that, upon execution of the program by a computer, cause the computer to perform the method of claim 16.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0048] The disclosure will now be described with reference to the drawings wherein:

[0049] FIG. 1 shows an exemplary embodiment of an apparatus for measuring the optical effect of an optical lens arranged in a measurement volume;

[0050] FIG. 2 shows an exemplary embodiment of a test structure recorded through a lens;

[0051] FIG. 3 shows an exemplary embodiment of a test structure recorded through a tilted spectacle lens;

[0052] FIG. 4 shows an illustration of beam paths through a transparent object;

[0053] FIG. 5 shows an illustration of beam paths through a lens;

[0054] FIG. 6 shows a lens composed of parameterizable surface elements;

[0055] FIG. 7 shows a schematic illustration of an apparatus for measuring the optical effect of an optical lens arranged in a measurement volume;

[0056] FIG. 8 shows a further exemplary embodiment of an apparatus for measuring the optical effect of an optical lens arranged in a measurement volume;

[0057] FIG. 9 shows a flow chart of a configuration of a method for measuring the optical effect of an optical lens arranged in a measurement volume;

[0058] FIG. 10 shows a detailed flowchart of a configuration of such a method;

[0059] FIG. 11 shows a flowchart of an exemplary embodiment of a calibration method;

[0060] FIG. 12 shows a schematic illustration of an eye;

[0061] FIG. 13 shows an image representation of a plan view of the eye with an image representation of the iris;

[0062] FIG. 14 shows a schematic illustration of an apparatus for measuring the cornea;

[0063] FIG. 15 shows the association or correlation of image features in images of the iris recorded from different viewpoints;

[0064] FIG. 16 shows a further correlation of image features; and

[0065] FIG. 17 shows a flowchart of an exemplary embodiment of a method for measuring the cornea.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0066] The apparatus 10 shown in FIG. 1 serves to determine the optical effect of an optical lens 100, in particular a spectacle lens. The apparatus 10 comprises a display device 20, which is configured to display a test structure 21. By way of example, this could be a screen or display and to be able to display different test structures.

[0067] The apparatus 10 further comprises an image capturing device 30 which is configured to capture image data of the test structure 21 from a plurality of viewpoints 31, 31′, 31″ by way of imaging beam paths 32 which pass through the lens 100. On the one hand, the imaging beam paths from the various viewpoints could be recorded successively by one camera, which is successively arranged at the various positions. However, a plurality of cameras are typically provided in order to capture the image data in parallel. It is understood that mixed forms may also be provided. By way of example, an image capturing device 30 can comprise a first camera 33 and a second camera 34, wherein the first camera 33 is configured to capture first image data from a first viewpoint 33 and the second camera 34 is configured to capture second image data from a second viewpoint 33″. The measurement volume 200 is located between the test structure 21, which is displayable on the display device 20, and the image capturing device 30.

[0068] The apparatus 10 further comprises a computing unit 40. By way of example, the computing unit 40 can be a computer, a microcontroller, an FPGA or the like. The computing unit 40 is configured to determine a three-dimensional shape of the lens 100 on the basis of the image data; and to calculate an optical effect of the lens 100 on the basis of the three-dimensional shape. Expressed differently, a two-stage procedure is proposed, in which the three-dimensional shape of the lens is initially determined and only then is the optical effect of the lens calculated from its three-dimensional shape.

[0069] This approach as per the present disclosure should be explained in more detail below with reference to FIGS. 2 to 6.

[0070] FIG. 2 shows a plan view of a test structure 21 of a display device 20 recorded through a lens 100. By way of example, this can be the test structure 21 as per FIG. 1, recorded by the camera 33 through the lens 100. The test structure 21 is reproduced in distorted fashion as a result of the lens 100. Conclusions about the optical effect of the lens 100 can already be drawn from such a deflection of the beams. However, it is only possible to make a statement about the effect of the lens 100 as a whole.

[0071] FIG. 3 shows a further exemplary image, which has been recorded by a camera.

[0072] However, in this case, the spectacles 101 with the optical lens 100 are arranged with a large tilt in the measurement region, and so the beam deflection caused by the optical lens 100 only reproduces the actual optical effect in a wearing position with limited accuracy.

[0073] FIG. 4 shows an exemplary illustration of beam paths through a transparent object such as a lens 100. The origin of the beam paths is a defined point 22 on the display device 21. The beam paths emanating from the defined point 22 enter the lens 101 at the surface 102 and emerge from the lens at the surface 103. Consequently, they pass through the lens 100. The light beams are refracted both at the entry surface 102 and at the exit surface 103. A different optical effect may arise, depending on the relative spatial position or orientation of the lens 100 with respect to the display device 20 and the image capturing device 30.

[0074] The inventors have recognized that such uncertainty or ambiguity of the optical effect can be resolved by virtue of recording the test structure from a plurality of viewpoints and consequently capturing a multiplicity of imaging beam paths (see also FIG. 1), from which, in turn, the optical properties of an interposed lens can be determined. Expressed differently, a system of equations with a multiplicity of equations can be set up for the imaging beam paths, which equations each establish an association between imaging beams, which pass through the lens 100 and enter the image capturing device from a plurality of viewpoints, and the known origins thereof on the display device 20. From this, it is possible, in turn, to determine the three-dimensional shape of the lens 100 and also, optionally, its refractive index or a brake-force distribution within the lens as well.

[0075] A simplified example of beam paths through a lens 100 is reproduced in FIG. 5. The image point 22 on the display device 20 is captured by the camera 34. The beam path 110 enters the lens at the point 104 on the front side 102 of the lens 100 and emerges from the lens at the point 106 on the back surface 103 of the lens. In simple terms, there is therefore an equation with two unknowns, the entry point 104 and the exit point 105 including the spatial orientation of the surface at these points, in the case of a measurement with only one camera. By virtue of the image capturing device 30 recording the test structure from further viewpoints, as indicated by the further camera 33, it is possible to capture further beam paths 111 and 112. In the case of the beam path 111, an exit point 104 coincides with the beam path 110 of the camera 34. In the case of the beam path 112, an entry point 105 coincides with the beam path 110 of the camera 34. Consequently, it is possible to set up a plurality of equations, from which it is possible to determine the properties of the lens 100 arranged between the display device 20 and the image capturing device 30.

[0076] To this end, the computing unit can be configured to model the lens 100, typically as a composed surface made of parameterizable surface elements, as shown in FIG. 6 in exemplary fashion. An orientation of the surface elements 106, 107 of the front and back surface 102, 103 can be determined from the deflection of the beams at the points 104 and 105. Optionally, a further division can be undertaken within the lens 100. By way of example, further interfaces can be determined within the lens 100. Optionally, the computing unit can further be embodied to determine a refractive index of the lens 100 or else spatial refractive index distribution of the lens.

[0077] Optionally, the apparatus can be embodied as an apparatus for measuring a spatial refractive index distribution of an optical lens arranged in a measurement volume. To this end, provision can typically be made of an interface which is configured to receive lens geometry data, which describe a three-dimensional shape of the lens. In this case, the shape of the lens need not be calculated; instead, it can serve as an input parameter for calculating the spatial refractive index distribution of the lens on the basis of the image data and the lens geometry data.

[0078] Referring to FIG. 5 and FIG. 6, the computing unit can be configured to determine three-dimensional shape of the lens on the basis of tracing back the light beams entering the image capturing device. The directions from which the light beams 110, 111, 112 enter the cameras 33, 34 of the image capturing device are known. To this end, the image capturing device, as still described below, can be calibrated. Consequently, the entering beams can be traced back proceeding from the respective camera 33, 34. The lens 100 is located in the beam path between the image capturing device or the respective cameras (with a known position) and the test structure (with a known position). Proceeding from a model of the lens 100, this model can successively be parameterized by the computing unit in such a way that the (known) test structure is imaged by the model of the lens 100 in such a way that the image data captured by the image capturing device arise. For the parameterization, it is possible, in particular, to adapt the alignments of the surface elements, represented here by the surface normals 129, 130, forming the lens surface and vary a distance 131 between the surface elements, and it is optionally possible to vary a refractive index n or a refractive index distribution within the lens.

[0079] FIG. 7 and FIG. 8 show further exemplary embodiments of an apparatus 10 for measuring the optical effect of an optical lens 100 arranged in a measurement volume. Corresponding assemblies are denoted by the same reference sign and not explained again in detail in order to avoid repetition.

[0080] FIG. 8 shows an exemplary embodiment in which the image capturing device 30 comprises two cameras 30, 31′. The cameras see a pattern or a test structure 21 from different viewing angles. The computing unit is embodied to reconstruct the test object 4 from the corresponding image data. To this end, it is possible to determine a gradient field from the surface elements or from the normals 129, 129′, as explained in FIGS. 5 and 6.

[0081] Light from defined sources at defined origins of the test structure 21 passes through the lens 100 and is captured by the image capturing device 30 from different viewing angles by means of a calibrated camera system. The refractive surfaces of the body are reconstructed from the images arising.

[0082] The principle works with one camera, two cameras or more cameras. Two cameras are used in an exemplary embodiment, as a good cost/use ratio can be obtained in this case. Even more cameras can be used to further increase the accuracy.

[0083] The image capturing device 30 or the cameras 31, 31′ is/are calibrated in such a way that a function is known, by means of which a unique chief light ray (camera ray) can be derived in 3D for each sensor coordinate from the origin and direction. This calibration can be carried out according to the related art. Alternatively, a known optical design of the camera and/or of an employed objective can be included in the model of the camera instead of the above-described camera calibration.

[0084] By way of example, the display device 20 can have self-luminous sources, such as light-emitting diodes arranged in an array, a TFT or LED display, a 3D display, laser sources, a polarization display, or else a collimated, selectively structured illumination unit. Light can also be shone on the display apparatus. By way of example, a display apparatus on which light is shone may have test charts (e.g., a point pattern or checker pattern), an in particular regular 3D pattern, an unknown feature-rich flat image (wherein positions can be estimated during the operation) or else an unknown feature-rich 3D scene (positions are estimated during the optimization).

[0085] The computing unit 40 can use further information for determining the three-dimensional shape. The reconstruction of the three-dimensional shape may in particular also be based on the known viewpoints or positions of the camera, from which the image data are captured, and a known position of the test structure. In the present example, the image data can be locations of the imaging of light beams, entering the cameras, on the camera detectors. The light beams entering the image capturing device can be calculated from the image data and the known viewpoints. A calibration of the image capturing device can serve as a basis for this.

[0086] Optionally, the computing unit 40 can further be configured to determine the three-dimensional shape of the lens taking account of one or more boundary conditions. By way of example, a contact point or stop 51 may be predetermined. The relative position of the lens 100 is known at this point and can be taken into account when determining the three-dimensional shape of the lens. Further, information such as the shape of a front and/or back surface of the lens, a refractive index or material, etc., may be predetermined. Optionally, the apparatus can be embodied to read information present on the lens, for example in the form of an engraving or a marker 140, and take this information into account when determining the three-dimensional shape and/or when calculating the optical effect.

[0087] A particularly advantageous application of the present disclosure lies in the measurement of spectacle lenses, in particular the measurement of progressive spectacle lenses—also known as varifocal spectacle lenses. Simpler spectacle lenses such as spherical, aspherical, toric or prismatic lenses can, however, likewise be measured using the apparatus proposed.

[0088] Optionally, the computing unit can be configured to calculate an ISO vertex power or a vertex power in a specified measuring appliance configuration in order to provide comparable data. By providing wearer-specific data, such as the distance of a pupil from the spectacle lens (vertex distance) and its relative position (e.g., face form angle or “as worn” pantoscopic angle), it is possible to calculate use vertex powers.

[0089] Optionally, a plurality of test objects can be measured simultaneously in the measurement space. In the case where a pair of spectacles with a left and a right spectacle lens is measured, the computing unit can be further embodied to determine a position and relative position of the spectacle lenses with respect to one another. From this, it is possible to calculate further information, such as the distance of the optical channels for example. A transparent body with zones of different effects can also be provided as a plurality of test objects. By way of example, this can be a pair of spectacles with two lenses or a lens with a plurality of zones—bifocal lens, trifocal lens or multifocal lens.

[0090] FIG. 9 shows a flow chart of a method 900 for measuring the optical effect of an optical lens, in particular a spectacle lens, arranged in a measurement volume, including the steps set forth below. In a first step 901, a test structure is provided for display on a display device. In a second step 902, image data of the test structure are captured from a plurality of viewpoints by way of imaging beam paths that pass through the lens. In a third step 903, a three-dimensional shape of the lens is determined on the basis of image data (and the known positions of the viewpoints and the display device relative to one another). In a fourth step 904, an optical effect of the lens is calculated on the basis of its three-dimensional shape. The calculation can be implemented for any usage situation. Consequently, the computing unit can be configured to calculate a first optical effect, corresponding to an ISO vertex power, and a second optical effect, corresponding to a usage situation by a user.

[0091] Optionally, the measuring method can be preceded by step 905 for calibrating the apparatus.

[0092] A corresponding method for calibrating the apparatus may, in turn, include the following steps: In a first calibration step, a test structure is provided on the display device. In a second calibration step, a first distance is set between the image capturing device and the display device and image data of the test structure are captured from the first distance by means of the image capturing device.

[0093] As shown in FIG. 8, provision can be made of a height adjustment device 150, which is configured to vary a distance between the image capturing device and the display device. In this context, the computing unit can be further configured to determine, on the basis of image data captured from different distances between the image capturing device and the display device, a beam direction of the light beams captured by the image capturing device.

[0094] In a further step of the method for calibrating the apparatus, a second distance can be set between the image capturing device and the display device and image data of the test structure are captured from the second distance by means of the image capturing device. From this, a direction of incident light beams, captured by the image capturing device, and corresponding image points in the image data can be determined in a further step.

[0095] FIG. 10 shows a detailed flow chart of an exemplary embodiment of a method 1000 for measuring the optical effect of a lens arranged in a measurement volume.

[0096] In a first step S1011, a test structure is displayed on the display device. By way of example, this can be an entire point or stripe pattern. In a further step S1012, image data of the test structure are captured by the image capturing device. In step S1013, it is possible to determine positions of features of the test structure, for example the positions of pattern points in the image data (corresponding to positions on a detector surface of the image capturing device). Here, there can be a camera calibration step S1001, as explained above or described in detail in FIG. 11, for example. In step S1014, it is then possible to determine the light beams incident in the image capturing device or the directions thereof. The light beams incident in the camera can be determined as a 3D vector from the camera images of the displayed pattern, as observed through the lens to be measured.

[0097] In a step S1021, a complete or partial pattern of a test structure can be displayed on the display device. In a further step S1022, image data of the test structure are captured by the image capturing device. In step S1023, pattern points can be associated with image points in the image data. In particular, it is possible to provide a sequence of different test patterns in order to resolve a possible ambiguity when associating pattern points with image points in the image data. Expressed differently, luminous spots in the image data captured by the image capturing device can be assigned to a position of the luminous points on the display device, and hence also to the calculated light beams, which were incident in the image capturing device. As an alternative or in addition thereto, the computing unit can be configured to determine neighborhood relationships from an overall pattern of a test structure.

[0098] In a step S1031, a planar illumination can be provided on the display device. By way of example, all pixels of the display device could display “white”. As a consequence, a contour of the lens could stand out and a contour of the lens can be determined in step S1032. In a step S1033, a relative position and dimensions of the lens can be determined on the basis of the captured contour. Expressed differently, a relative position of the lens in the measurement volume can be determined in a simple manner.

[0099] In step S1041, there can be a calculation of a “best fitting” parameterizable lens. Typically, a “best fitting” parameterizable lens, which could lie in the measurement volume of the appliance, can be ascertained by back propagation of the camera light beams. A parameterizable lens is understood to mean a lens that can be described by few parameters such as radius, thickness or the refractive index. These include spherical and toric lenses, for example. Toric lenses are a general compromise, which may be applied here. In a more specific exemplary embodiment, it may be sufficient to define individual “toric zones” on the lens and only describe the spectacle lens there. By way of example, a first of these zones could be a “far region” of a progressive lens. By way of example, a second of these zones could be a “near region” of a progressive lens. In addition to the location of the lens or the individual surfaces, further parameters could be the radii, the thickness and the refractive index.

[0100] In step S1042, a “best fitting” gradient surface of the front and/or back surface of the lens can be determined by inverse ray tracing of the camera rays. Consequently, a surface of the “best fitting” parameterizable lens determined in step S1041 can be described as a gradient surface and the gradients at the locations of the beam passage can be varied in such a way that the positions of the luminous points on the display device are impinged perfectly by back propagation of the camera rays. In simple terms, the three-dimensional shape of the lens is therefore adapted in such a way that the light beams received by the image capturing device and the associated beam sources fit together on the display device.

[0101] In step S1043, a front and/or back surface of the lens can be obtained by integration from the gradient surface. Expressed differently, a (continuous) new surface is determined from a piecewise gradient surface or a gradient surface determined for surface elements. Here, this could be the front surface or the back surface of the lens.

[0102] According to step S1044, steps S1042 and S1043 can be repeated iteratively. By way of example, the steps could be repeated until a quality criterion has been satisfied. Optionally, if a sufficient quality cannot be reached, step S1041 can also be included in the iteration loop, in order to take account of alternative lens geometries. A three-dimensional shape of the lens can be available as a result of the iteration.

[0103] In a further exemplary embodiment, which may assist with the understanding of the disclosure, a shape of the lens can be predetermined and, instead, a spatial refractive index distribution within the lens can be iteratively determined in analog fashion.

[0104] One or more variables can be subsequently determined from the determined three-dimensional shape (optionally including the refractive index). A use value, in particular a user-specific use value, can be calculated in step S1052. To this end, wearer-specific data, such as a distance between cornea and apex, can be provided in step S1051. An ISO vertex power can be determined in step S1053. A vertex power in an appliance configuration can be determined in step S1054.

[0105] If a plurality of lenses or spectacle lenses were arranged in the measurement volume at the same time, it is optionally possible to determine additional parameters, such as the spacing of the progression channels.

[0106] It is understood that the aforementioned steps can be carried out by the computing unit and that the latter can be configured accordingly for the purposes of carrying out the steps.

[0107] FIG. 11 shows a flow chart of an exemplary embodiment of a method 1100 for calibrating an apparatus for measuring individual data of an optical lens arranged in a measurement volume, which may assist with the understanding of the disclosure. The calibration can serve, in particular, to provide a set of functions, which assigns a beam direction such as a 3D vector to an image point—typically each image point—of the image data captured by the image capturing device, the 3D vector describing a beam direction or a light beam entering the image capturing device. Such a set of functions can have the following form:

[00001]$\stackrel{.fwdarw.}{r}\left(x,y\right)=\left(\begin{array}{c}{x}_0\\ y_0\\ 0\end{array}\right)+\alpha \left(\begin{array}{c}dx\\ dy\\ 1\end{array}\right)$

where (x.sub.0, y.sub.0, 0) describes a point of the light beam in a reference plane of the image capturing device, typically a point of the light beam in a reference plane in the lens system of a camera of the image capturing device, and (dx, dy, 1) describes the direction vector of the incident beam. Consequently, the set of functions consists of four functions: x.sub.0(x,y), y.sub.0(x,y), dx(x,y), and dy(x,y), where x and y describe the pixel coordinates in image data of the image capturing device, a camera in this case.

[0108] Such a set of functions can be determined by virtue of a test structure, e.g., a point pattern, being displayed on the display device and being observed by the cameras of the image capturing device from different distances. For this purpose, the apparatus as illustrated in FIG. 8 in exemplary fashion can comprise a height adjustment device 150, which is configured to vary a distance between the image capturing device and the display device.

[0109] In the method shown in FIG. 11, steps S1101 to S1104 can respectively correspond to steps S1011 to S1014 in FIG. 10, which were described above. Steps S1111 to S1113 shown in FIG. 11 can each correspond to the above-described steps S1021 to S1023. However, a loop is provided, with a distance between the image capturing device and the display device being varied in step S1121. As shown in FIG. 8, a variation in the distance changes a direction of the incident light beams. The computing unit can be configured to determine a direction of the incident light beams on the basis of the image data captured at the first distance and the image data captured at the second distance. It is understood that this determination can further depend on a relative position between the display device and the image capturing device and on the variation of the distance.

[0110] As shown in FIG. 11, an interpolation of 3D light beams of a plurality of image points or pixels, typically for each image point or pixel, of the image capturing device can be undertaken in a step S1131. Subsequently, the variables x.sub.0, y.sub.0, dx, and dy can be determined for the image points in step S1132. In a next step S1133, a polynomial fit can be applied to the variables x.sub.0, y.sub.0, dx, and dy. As a result, this can be used to determine a set of functions which assigns a direction of an incident light beam to each image point of the image data captured by the image capturing device.

[0111] Optionally, a relative spatial position of a contact point 51, as illustrated in exemplary fashion in FIG. 8, can be determined in a manner analogous to the camera calibration. To this end, the display device 20 can be configured to display a single color background or a background with a constant brightness as a test structure. The image capturing device captures the display without an inserted lens. In this case, the contact points lead to shadow (e.g., circle in the case of contact spheres), which is captured by the image capturing device and contained in the image data. The computing unit can be embodied to determine a position of the contact points 51 on the basis of these image data. In turn, these can be used as boundary conditions when determining a three-dimensional shape of a lens. When observing a contact point by at least two cameras, its central point (the relative position) can be ascertained by an intersection of the camera rays. The position of the contact points can be used to assign to an algorithm for determining the shape of the spectacle lens an expected value for the position of the latter in the measurement volume. An advantage of this configuration consists in an improved accuracy.

[0112] It is understood that the explanations made above can apply accordingly to the exemplary embodiments below, and vice versa. To avoid repetition, further aspects, in particular, are intended to be discussed below. Features of the aforementioned exemplary embodiments and the exemplary embodiments below can advantageously be combined with one another.

[0113] The inventors have recognized that the concepts described herein can also be advantageously used for measuring the cornea. FIG. 12 shows a schematic sectional illustration of an eye 1200. The eye comprises a cornea 1201, an Iris 1202, a pupil 1203, and the lens 1204. FIG. 13 shows a plan view of the eye with an image representation of the iris 1202 and the pupil 1203. However, a substantial contribution to the refractive error does not come from the lens 1204 in this case, but from the cornea 1201. A substantial contribution to the refractive error of a subject can be due to corneal curvature. It would therefore be desirable to be able to objectively determine the shape of the cornea.

[0114] FIG. 14 shows a schematic illustration of an apparatus for measuring the cornea of a subject as per a further aspect of the present disclosure. The apparatus may comprise the following: an image capturing device (30), which is configured to capture image data of an iris (1202) of the subject from a plurality of (known) viewpoints by way of imaging beam paths (32) which pass through the cornea (1201); and a computing unit (40). The computing unit is configured to: provide a mathematical model of an anterior eye section of the subject including a mathematical model of the cornea and the iris; identify and register image features of the iris which are present in a plurality of images of the image data; determine deviations between actual positions of the image features of the iris in the images captured from the plurality of viewpoints and expected positions of the image features of the iris in the images captured from the plurality of viewpoints taking into account the mathematical model of the cornea and the relative position of the iris; adapt parameters of the mathematical model of the cornea in such a way that the deviations are minimized; and determine a measured variable of the cornea from the adapted mathematical model of the cornea.

[0115] The image capturing device 30 can again have the same or similar configuration as described for FIG. 1. The image capturing device captures image data of the iris 1202, with the beam paths passing through the cornea 1201. To capture the image data from different known viewpoints, a camera 33 can be successively positioned at different known positions. To this end, provision can be made of a positioning device (not shown). As an alternative or in addition thereto, provision can be made of a plurality of cameras 33, 34, which capture the image data in parallel. An advantage of the parallel capture consists of the eye of the user not moving between the various measurements.

[0116] The inventors have recognized that the cornea 1201 situated between the iris 1202 and the image capturing device 30 can also be calculated without knowledge about how the iris 1202 looks. The iris 1202 has an unknown structure or an unknown pattern. However, the iris 1202 is usually very structured. The inventors have recognized that a multiplicity of image features of the iris can be identified and subsequently evaluated in respect of their position in a plurality of images of the image data, which were recorded from different positions. To this end, a system of equations can be set up from the imaging beam paths 32 which are captured at the respective known positions; the shape of the cornea 1201 can be calculated therefrom.

[0117] FIG. 15 and FIG. 16 show the association or correlation of unknown image features in images of the iris recorded from different viewpoints. The left image in FIG. 15 shows a first image representation 1500a of the iris 1202—through the cornea—from a first position, for example recorded by the camera 33 in FIG. 14. The right image in FIG. 16 shows a second image representation 1500b of the iris 1202—through the cornea—from a second position, for example recorded by the camera 34 in FIG. 14. Since the iris is usually a very structured area, it is possible to determine a correlation 1502 between the same initial points 1501a and 1501b of the iris 1201. A further example of the corresponding points is specified by the reference signs 1503a and 1503b, linked by 1502′. Such an association 1500 can be undertaken for a multiplicity of images 1500a to 1500d and image points, as shown in FIG. 16.

[0118] On the basis of this correlation or association analysis, it is possible to reconstruct a multiplicity of beam paths, as shown in FIG. 14. As is evident from the shown beam paths 32, the light beams emanating from the same image point on the iris 1202 pass through the cornea 1201 at different points and are captured at different locations by the image capturing device 30. Should now the same starting point be identified in the image representations, it is possible to make statements about the cornea 1201, which is located between the starting point on the iris 1202 and the entry into the cameras 33, 34, as described above with reference to FIG. 14.

[0119] FIG. 17 shows a flowchart of an exemplary embodiment of a method for measuring the cornea of a subject, which may assist with the understanding of the disclosure. The camera system can be calibrated in an optional preceding step. However, the manufacturer may have already carried out the calibration. In a first step 1701, image data of an iris of the subject can be captured from a plurality of viewpoints by way of imaging beam paths that pass through the cornea. In a second step 1702, a mathematical model of an anterior eye section of the subject with a mathematical model of the cornea (and the relative position of the iris with respect to the cornea) can be provided. In a third step 1703, image features of the iris which are present in a plurality of images (typically in all images) of the image data can be identified and registered (or assigned in the images). In a fourth step 1704, it is possible to determine deviations between the actual positions of the image features of the iris in the images captured from a plurality of viewpoints and the expected positions of the image features of the iris in the images captured from a plurality of viewpoints taking into account the mathematical model of the cornea and the relative position of the iris. In a fifth step 1705, parameters of the mathematical model of the cornea can be adapted in such a way that the deviations are minimized. Steps 1703 and 1704 can typically be repeated iteratively. In a sixth step, it is now possible to determine a measured variable of the cornea from the adapted mathematical model of the cornea. By way of example, a refractive power or an astigmatism can be evaluated.

[0120] In conclusion, the solutions disclosed herein can facilitate, in particular, a simplified contactless measurement of lens elements arranged in a measurement volume or else a contactless measurement of the cornea, in particular with a reduced impairment of a light-sensitive user, in the field of ophthalmic optics.

[0121] The foregoing description of the exemplary embodiments of the disclosure illustrates and describes the present invention. Additionally, the disclosure shows and describes only the exemplary embodiments but, as mentioned above, it is to be understood that the disclosure is capable of use in various other combinations, modifications, and environments and is capable of changes or modifications within the scope of the concept as expressed herein, commensurate with the above teachings and/or the skill or knowledge of the relevant art.

[0122] The term “comprising” (and its grammatical variations) as used herein is used in the inclusive sense of “having” or “including” and not in the exclusive sense of “consisting only of” The terms “a” and “the” as used herein are understood to encompass the plural as well as the singular.

[0123] All publications, patents and patent applications cited in this specification are herein incorporated by reference, and for any and all purposes, as if each individual publication, patent or patent application were specifically and individually indicated to be incorporated by reference. In the case of inconsistencies, the present disclosure will prevail.