Calibration procedure for a camera-based measuring device for human eye diagnostics

Abstract

Calibration method for a camera-based measuring device (12) for human eye diagnostics, comprising: Positioning (40) a calibration object (22) simulating an anterior eye region on a holder (20) arranged to be movable relative to a camera (16) of the measuring device (12) in a first direction within a field of view of the camera (16), wherein the first direction represents a direction towards and away from the camera (16) substantially along a camera axis, wherein the calibration object (22) is designed in the form of a dome; first controlling (42) a motorized drive unit connected to the holder (20) in a drive connection in order to move the holder (20) with the calibration object (22) positioned thereon in the camera field of view relative to the camera (16) along the first direction at least once over a position of highest image sharpness of an image of the calibration object (22) recorded by the camera (16); and Determining (44) a ratio between an actual size and an image size of the calibration object (22) on the basis of a camera image of the calibration object (22) recorded at the position of highest image sharpness.

Description

Technical FieldThe present disclosure generally relates to a calibration method for a camera-based measurement device of human eye diagnostics. It relates in particular to a method for the camera-based measuring device to calibrate it for preoperative diagnostic measurements in cataract surgery.BackgroundIn the (laser-refractive) cataract surgery, the natural, opaque lens of a patient's eye is replaced by an artificial lens. For this purpose, the anterior chamber of the patient's eye is opened, the natural lens is removed and finally a synthetic lens is inserted at the position of the natural lens.In the context of preoperative diagnostics for cataract surgery, a high-resolution reference image of the patient's eye to be treated is recorded for the measurement thereof. The recorded reference image contains, for example, scleral blood vessels, the limbus and landmarks on the iris of the patient's eye. From the reference image, the position of cuts to be made and the positioning of the artificial lens to be inserted can be determined. The reference image forms the basis for the calculation of a cataract operation plan.The reference image of the patient's eye is recorded with a camera. By means of suitable image processing, properties of the patient's eye, such as the limbus diameter, can be read out from the reference image. For this purpose, a suitable calibration is necessary in order to be able to reliably determine the actual properties, such as the real limbus diameter of the patient's eye, from the properties of the patient's eye depicted in the reference image.The document DE 10 2015 001 874 A1 discloses a calibration method for a device for determining the distance of the user's eyes from an image recording device. As test objects for calibration, inter alia lenses with different radii of curvature can be used.Tear-offAn object of the present invention is to provide a reliable calibration for determining properties of a patient's eye or a test object from a recorded reference image thereof.According to certain embodiments, a calibration method for a camera-based measuring device of human eye diagnostics is provided. The calibration method comprises positioning a calibration object simulating an eye front region on a holder arranged such that it can be moved in a first direction relative to a camera of the measuring device within a field of view of the camera. The first direction represents a direction toward and away from the camera substantially along a camera axis (such as along the optical axis of the camera). The calibration method further comprises a first control of a motorized drive unit which is in drive connection with the holder in order to move the holder with the calibration object positioned thereon in the camera field of view relative to the camera along the first direction at least once over a position of maximum image sharpness of an image of the calibration object recorded by the camera. Furthermore, the calibration method comprises ascertaining a ratio between an actual size and an imaging size of the calibration object on the basis of a camera image of the calibration object recorded at the position of highest image sharpness.The calibration, i.e. the determination of the calibration ratio between the actual size and the imaging size of the calibration object, takes place on the basis of the camera image recorded at the position of highest image sharpness. The high image sharpness enables a high accuracy of the determination of the image size from the camera image, which is carried out, for example, by means of a suitable image processing program (automated).According to a first aspect, the calibration object simulating an eye front region is designed in the form of a dome, for example in the form of a hemisphere. Thus, the dome-shaped surface of the calibration object can simulate, for example, a region of the human cornea. As the actual variable, at least one predetermined diameter of the calibration object can be present, which is related to a diameter determined as the imaging variable from the camera image recorded at the position of highest image sharpness. The predetermined diameter of the calibration object can correspond, for example, to a usual limbus diameter. A value between 7.0 mm and 10.0 mm can be provided for the predetermined diameter.According to a second aspect, the camera comprises a plurality of infrared light sources for illuminating an object within the field of view of the camera and a sensor for detecting reflected infrared light.In order to reduce uncertainties in the determination of the image size and thus to further improve the accuracy of the calibration, the first control of the calibration method can comprise controlling the drive unit such that the holder with the calibration object positioned thereon in the camera field of view is moved relative to the camera one or more times back and forth in each case and in each case over the position of highest image sharpness. In this case, it can be provided to determine the image size in each case from a plurality of camera images recorded in the position of highest image sharpness (for example from each of these camera images of highest image sharpness). An averaged image variable can be calculated from the plurality of determined image variables and used to determine the calibration ratio.The human eye has specific properties for each patient. For example, limbus diameter may vary between different patients. In order to take account of such variations in the calibration, it can be provided that the calibration method comprises repeating the steps of the first control and the determination in each case with a calibration object of different size positioned on the holder in the camera field of view.The holder can also be arranged such that it can be moved relative to the camera in a second direction running transversely to the first direction. At least in this case, the step of positioning may include positioning a plurality of calibration objects of different sizes side by side on the holder in the second direction. Further, the calibration method may include second controlling the drive unit to sequentially bring the calibration objects positioned on the holder to a position within the field of view of the camera. A sequence of the first control and the second control of the drive unit in alternation thus makes it possible to determine the image variable from a camera image recorded in the position of highest image sharpness first for a first and then for the further calibration objects arranged on the holder along the second direction in succession.For the calculation of an actual object size of an object (for example a test object or a patient's eye) different from the plurality of calibration objects, it may be necessary to determine the ratio of imaging size to actual size for an imaging object size different from the determined imaging sizes of the calibration objects. For this purpose, it can be provided that the calibration method comprises a determination, in particular by inter- or / and extrapolation methods, of a calibration function on the basis of the determined relationships between actual size and imaging size of all calibration objects, so that the calibration function provides an actual object size for each of a plurality of different imaging sizes of an object.Brief Description of the DrawingsAdditional features, advantages and components of the present invention can be taken from the following description of the attached drawings, in which: FIG. 1 shows a schematic block diagram of an exemplary embodiment of an arrangement for carrying out a calibration method for a camera-based measuring device of human eye diagnostics; and FIG. 2 shows a flow diagram of an exemplary embodiment of a calibration method for a camera-based measuring device of human eye diagnostics.DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTSFIG. 1 shows a block diagram of an exemplary embodiment of an arrangement, generally designated 10, for carrying out a calibration method (for example for carrying out the calibration method according to FIG. 2 ) for a camera-based measuring device 12 of human eye diagnostics. The arrangement 10 comprises the camera-based measuring device 12 and a calibration station 14.The camera-based measuring device 12 comprises a camera 16, the optical axis of which is indicated by the dashed line denoted by OA in FIG. 1. A focal plane F denotes an x-y plane extended transversely to the optical axis OA, in which images recorded by the camera 16 have a highest (maximum) image sharpness. A distance f between a front lens (not shown) of the camera 16 and the focal plane F is referred to below as the working distance of the camera 16.In the embodiment shown in FIG. 1, it is contemplated that camera 16 includes a plurality of infrared (IR) light sources (such as IR LEDs) (not shown) for illuminating an object within the field of view of camera 16, and a sensor (not shown) for detecting reflected IR light. Alternatively or additionally, the camera 16 can be provided with at least one light source for illuminating the object with light in the visible wavelength range and a sensor for detecting reflected light in the visible wavelength range.The camera-based measuring device 12 according to the exemplary embodiment shown in FIG. 1 further comprises a processing unit 18 for evaluating images recorded by the camera 16, in particular with regard to predetermined features. The evaluation can be carried out by means of a suitable image processing program. The processing unit 18 can furthermore comprise a display device (not shown) on which images recorded by the camera 16 and / or results of the evaluation of the images are displayed.The calibration station 14 comprises a holder 20 which is designed for arranging a plurality of calibration objects, generally designated by 22. According to the exemplary embodiment shown in FIG. 1, three calibration objects 22A, 22B, 22C mounted on a carrier 24 are received by the holder 20 or arranged on the holder 20. In another exemplary embodiment, it can be provided that the holder 20 is designed to hold or arrange one of three different numbers of calibration objects 22 (e.g. only one single calibration object 22). The holder 20 can be designed, for example, as a slide that can be moved at least in one direction.The calibration objects 22 reproduce a front region of a human eye. For this purpose, the calibration objects 22 illustrated in FIG. 1 are designed in the form of a hemisphere. The calibration objects 22 have different diameters which lie in the range of values of typical limbus diameters of the human eye. It can be provided, for example, that the three calibration objects 22A, 22B, 22C each have a diameter of 9.5 mm, 8.5 mm, 7.5 mm.In the present exemplary embodiment, the calibration objects 22 (produced, for example, from blackened glass) cancel one another out in color from the carrier 24 (produced, for example, from white plastic material). Thus, when illuminated with IR light or light in the visible wavelength range, the calibration objects 22 also cancel one another in color in an image recorded by the camera 16. By means of the processing unit 18, the calibration objects 22 or their contours can be identified in an image recorded by the camera 16.The calibration station 14 further comprises a drive unit 26 which is in a mechanical drive connection, indicated at 28, with the holder 20. The drive unit 26 is designed for moving or adjusting the position of the holder 20 at least along a first direction referred to as the z-direction. To this end, the drive unit 26 may include, for example, an electric motor drive unit.The drive unit 26 is controlled by a control unit 30 of the calibration station 14. The control unit 30 is, for example, program-controlled and can, for this purpose, contain a memory 32 with instructions stored therein, according to which the holder 20 is to be moved. It can be provided that the processing unit 18 of the measuring device 12 and the control unit 30 of the calibration station 14 are provided as a single component. At least in this case, the control unit 30 can be in electrical signal connection with the camera 16 in order to control illumination and / or image recording by means of the camera 16.As schematically shown in the arrangement 10 shown in FIG. 1, the calibration objects 22 arranged on the holder 20 are positioned within a plane transverse to the optical axis OA of the camera 16. During a movement of the holder 20 in the z direction, the calibration objects 22 are moved in the direction of the camera 16. If a distance between the calibration objects 22 and the front lens of the camera 16 corresponds to the working distance f, the calibration objects 22 are located within the focal plane F of the camera 16, i.e. in the position of highest image sharpness. The calibration objects 22 can be moved into and out of a field of view (not shown in FIG. 1 ) of the camera 16 by moving the holder 20 relative to the camera 16 (manually or by means of the drive unit 26) along a second direction (the x direction) running transversely to the optical axis.FIG. 2 shows the steps of an exemplary embodiment of a calibration method for a camera-based measuring device of human eye diagnostics, for example for the camera-based measuring device 12 described with reference to FIG. 1.In a first step 40, at least one calibration object simulating an eye front region is positioned within a field of view of a camera. In this case, provision can be made for the calibration objects 22 described with reference to FIG. 1 to be positioned along the x-direction running transversely with respect to the optical axis of the camera 16 (e.g. on the carrier 24 aligned in the x-direction), such that one of the calibration objects 22 arranged on the holder 20 (for example the calibration object 22A) is located within the field of view of the camera 16, while the other of the calibration objects 22 recorded by the holder 20 (for example the calibration objects 22B and 22C) are positioned outside the field of view of the camera 16. In particular, the calibration object 22A positioned within the field of view of the camera 16 may be disposed on the optical axis of the camera 16.In a following step 42 of the calibration method according to FIG. 2, a motorized drive unit 26 (see FIG. 1 ) is controlled in order to move the calibration objects 22 relative to the camera along a first direction. For example, the calibration objects 22 are moved in the z direction in the direction toward the camera 16 by means of the drive unit 26 comprised by the calibration station 14 according to FIG. 1. A distance between the front lens of the camera 16 and the calibration object 22A positioned within the field of view of the camera 16 is decreased beyond the working distance f of the camera 16. Before and / or after the described movement, the calibration objects 22 are moved counter to the z direction in the direction away from the camera 16. The camera 16 is configured to continuously capture images of the calibration object 22A positioned within the field of view of the camera 16, at least during a phase of movement.It can be provided to control the movement of the calibration objects 22 along the first direction within a predefined interval by an expected working distance f of the camera 16. For example, the expected working distance f may be about 17 cm and the distance between the front lens of the camera 16 and the calibration object 22A positioned within the field of view of the camera 16 may be varied within the interval 17 cm + / - 5 cm.In a further step 44, a first calibration ratio between an actual size and an imaging size of the (first) calibration object 22A positioned in the field of view of the camera 16 is determined on the basis of at least one camera image of the calibration object 22A recorded at the position of highest image sharpness. For this purpose, the camera image or the camera images are first determined, during the recording of which the calibration object 22A is located in the focal plane F of the camera 16.For this purpose, for example, a gradient between the image of the calibration object 22A and the region of the carrier 24 surrounding the calibration object 22A (see FIG. 1 ) can be evaluated. Alternatively or additionally, a circumference or a radius of the reflections of the illumination means of the camera 16 illuminating the calibration object 22A visible in the image of the calibration object 22A can be determined. In the focal plane F of the camera 16, the image sharpness of the images recorded by the camera is maximized. Accordingly, the evaluation of the camera image of maximum image sharpness shows a maximum gradient or a minimum reflection range or reflection radius.From the camera image or from the camera images of maximum image sharpness, imaging features can be read out, such as, for example, an imaging size of the calibration object 22A positioned within the field of view of the camera 16 being determined. In particular, a diameter of the calibration object 22A is determined as the image variable from the camera image or from the camera images of maximum image sharpness and is related to the actual diameter (actual size) of the calibration object 22A.If the calibration object 22A is moved by means of the drive unit a plurality of times beyond the position of maximum image sharpness (for example by a plurality of methods in the z direction and counter to the z direction), an imaging variable can be determined from each camera image of maximum image sharpness recorded during this. From the determined image variables, an averaged image variable of the calibration object 22A can be calculated in order to use this for determining the first calibration ratio (step 44).It can be provided that the evaluation of the camera images with regard to the position of highest image sharpness and / or with regard to the image size of the calibration object 22A is carried out by means of a suitable image processing program. The processing unit 18 included in the camera-based measurement device 12 of FIG. 1 may include such an image processing program.In a further (optional) step 46, the motorized drive unit 26 (see FIG. 1 ) is controlled in order to move the calibration objects 22 relative to the camera 16 along a second direction transverse to the optical axis OA of the camera 16. Thus, the calibration object 22A shown in FIG. 1 positioned in the field of view of the camera 16 is moved out of the field of view of the camera 16 in the x direction and the calibration object 22B arranged next to the calibration object 22A opposite to the x direction is moved into the field of view of the camera 16. Provision can be made for the calibration objects 22 to be moved along the second direction until the calibration object 22B is positioned on the optical axis OA of the camera 16. For this purpose, the calibration objects 22 can be positioned, for example, with a defined distance (for example known distance between their peak coupling points) along the second direction.With reference to FIG. 1, it is provided that the method of the calibration objects 22 according to steps 42 and 46 can be carried out in accordance with stored instructions by the controller 30. Alternatively, for example, the method according to step 46 may be carried out manually.In the subsequent step 48, the above-described step 42 of the method of the calibration objects along the first direction is repeated with the calibration objects in the positioning present after step 46. Further, in step 50, a second calibration ratio is determined for the calibration object positioned in the field of view of the camera (such as calibration object 22B of FIG. 1 ). With reference to FIG. 1, after execution of steps 40 to 50 of the method according to FIG. 2, a (e.g. averaged) calibration ratio for the calibration objects 22A and 22B is respectively present. In an alternative exemplary embodiment, it may be provided to ascertain a calibration ratio for a larger number of calibration objects 22 (e.g. for the three calibration objects 22A, 22B and 22C), in each case.If steps 46 to 50 of the method according to FIG. 2 are carried out (as indicated by the dashed arrows), a calibration function can be determined in a further step 52 on the basis of the plurality of determined calibration ratios between actual size and imaging size of the calibration objects. For this purpose, inter- and / or extrapolation methods are used in order to provide an actual object size for each of a plurality of different image sizes of an object. For example, linearly interpolation can be performed between determined calibration ratios. It can be provided that the processing unit 18 according to FIG. 1 is configured to determine such a calibration function.The calibrated measuring device 12 can thus be used for measuring objects of varying actual size. Thus, properties of a test object (e.g. in the manner of the calibration objects 22 according to FIG. 1 ) or of a patient's eye can be evaluated. For example, it may be provided to determine a limbus diameter or a pupil diameter from an image recorded by means of the camera 16 of the measuring device 12.

Claims

Calibration method for a camera-based measuring device (12) of human eye diagnostics, comprising: positioning (40) a calibration object (22) simulating an eye front region on a holder (20), arranged such that it can be moved in a first direction relative to a camera (16) of the measuring device (12), within a field of view of the camera (16), wherein the first direction represents a direction towards the camera (16) and away therefrom substantially along a camera axis, wherein the calibration object (22) is designed in the form of a dome; first controlling (42) a motorized drive unit which is in drive connection with the holder (20) in order to move the holder (20) with the calibration object (22) positioned thereon in the camera field of view relative to the camera (16) along the first direction at least once across a position of maximum image sharpness of an image of the calibration object (22) recorded by the camera (16); and determining (44) a ratio between an actual size and an imaging size of the calibration object (22) on the basis of a camera image of the calibration object (22) recorded at the position of maximum image sharpness.Calibration method for a camera-based measuring device (12) of human eye diagnostics, comprising: positioning (40) a calibration object (22) simulating an eye front region on a holder (20), arranged such that it can be moved in a first direction relative to a camera (16) of the measuring device (12), within a field of view of the camera (16), wherein the first direction represents a direction towards the camera (16) and away therefrom substantially along a camera axis, wherein the camera (16) comprises a plurality of infrared light sources for illuminating an object within the field of view of the camera (16) and a sensor for detecting reflected infrared light; first controlling (42) a motorized drive unit which is in drive connection with the holder (20) in order to move the holder (20) with the calibration object (22) positioned thereon in the camera field of view relative to the camera (16) along the first direction at least once across a position of maximum image sharpness of an image of the calibration object (22) recorded by the camera (16); and determining (44) a ratio between an actual size and an imaging size of the calibration object (22) on the basis of a camera image of the calibration object (22) recorded at the position of maximum image sharpness.The calibration method according to claim 1 or 2, wherein the first control (42) comprises controlling the drive unit (26) in order to move the holder (20) with the calibration object (22) positioned thereon in the camera field of view one or more times in each case back and forth relative to the camera (16) along the first direction and in each case across the position of highest image sharpness.Calibration method according to one of the preceding claims, comprising: repeating the steps of the first control (42) and of the determination (44) in each case with a calibration object (22) of different size positioned on the holder (20) in the camera field of view.Calibration method according to claim 4, wherein the holder (20) is arranged such that it can be moved relative to the camera (16) also in a second direction running transversely to the first direction, wherein the step of positioning comprises positioning (46) a plurality of calibration objects (22) of different sizes next to one another in the second direction on the holder (20), and wherein the method comprises second controlling (48) the drive unit (26) in order to bring the calibration objects (22) positioned on the holder (20) successively into a position within the field of view of the camera (16).Calibration method according to Claim 4 or 5, further comprising: determining (52) a calibration function on the basis of the determined relationships between actual size and imaging size of all calibration objects, wherein the calibration function provides an actual object size for each of a plurality of different imaging sizes of an object.Calibration method according to Claim 6, wherein the calibration function is determined on the basis of the ratios determined between the actual size and the imaging size of all calibration objects by inter- or / and extrapolation methods.

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