Procedure for testing an OCT device and test object

Abstract

Method for testing an OCT device (14) in which the axial resolution, the axial depth of the measurement window, the OCT roll-off, the numerical aperture, the OCT sensitivity, the signal-to-noise ratio and / or the image field curvature of the OCT device (14) are determined, in which a first test object (27) is arranged in an OCT beam path (16) of the OCT device (14), wherein the first test object (27) comprises a layer structure (29) of a plurality of transparent layers and an entrance body (30), wherein OCT light emitted from the OCT device (14) enters the entrance body (30) via an entrance surface and propagates through the entrance body (30) to the layer structure (29), and wherein the entrance surface of the entrance body (30) is shaped as a lens surface (31), and wherein the The lens surface (31) shaped entrance surface of the focus of the OCT beam path (16) is positioned within the layer structure (29).

Description

[0001] The invention relates to a method for testing an OCT device and a test object.

[0002] Optical coherence tomography (OCT) is an imaging technique in which OCT light is directed onto an object, particularly human tissue. Scattering centers within the object are inferred from the reflected portions of the OCT light. This is achieved by superimposing the reflected portions of the object's light path with a reference beam path to generate an interference signal. OCT image information is then obtained by analyzing this interference signal.

[0003] In OCT measurements, the axial measurement depth is limited. For a high signal strength of the interference signal, it is necessary to correctly adjust the reference plane and the focus of the OCT beam path. The reference plane is defined as a plane in the OCT beam path where the length of the object beam path and the length of the reference beam path coincide. This reference plane can have a depth position z = 0 mm within the so-called OCT measurement window.

[0004] Verifying OCT devices has traditionally been a complex and time-consuming process. To determine how signal strength changes with increasing distance from the reference plane, a test object can be placed in the object beam path, and the OCT device focused on it. If, with the measurement setup otherwise unchanged, the optical path length in the reference beam path is altered, the signal strength of the interference signal obtained from the test object changes depending on the axial distance between the reference plane and the flat surface. This change in signal strength is known as OCT roll-off.

[0005] In such measurements, the signal strength of the interference signal is often greater than in normal OCT measurements, which can lead to oversaturation of the OCT signal. To avoid oversaturation, the measurement can be performed with reduced intensity of the OCT light in the object beam path, but this has the disadvantage that the conditions during the verification measurement are not identical to those of a normal OCT measurement.

[0006] To determine the numerical aperture, focus position, and lateral resolution of the OCT device, a test object can also be placed in the object beam path so that the OCT device is focused on the object. If the test object is then moved axially while the reference arm remains at a constant length, both the distance from the reference plane and the distance from the focus position of the object beam path change. The resulting change in the signal strength of the interference signal can be measured. Here, too, the intensity of the object beam path must be regularly reduced to avoid oversaturation of the interference signal.

[0007] EP 3 182 062 B1 discloses a calibration method for determining the relationship between the control signal of a scan mirror and the lateral position of the OCT beam. DE 10 2018 207 827 B3, DE 10 2018 010 287 A1, DE 10 2021 113 074 B3 and US 2011 / 0181836 A1 disclose model eyes that can be used to demonstrate or practice the use of an OCT device for examining the retina.

[0008] The invention is based on the objective of presenting a method for testing an OCT device and a test object with which these disadvantages are avoided. This objective is achieved by the features of the independent claims. Advantageous embodiments are specified in the dependent claims.

[0009] In the inventive method for testing an OCT device, the axial resolution, the axial depth of the measurement window, the OCT roll-off, the numerical aperture, the OCT sensitivity, the signal-to-noise ratio, and / or the image field curvature of the OCT device are determined. A first test object is arranged in an OCT beam path of the OCT device. The first test object comprises a layered structure of a plurality of transparent layers and an entrance body. OCT light emitted by the OCT device enters the entrance body via an entrance surface and propagates through the entrance body to the layered structure. The entrance surface of the entrance body is shaped as a lens surface. The lens-shaped entrance surface positions the focus of the OCT beam path within the layered structure.

[0010] The invention proposes using interference signals obtained from the layer structure of the first test object for the verification of the OCT device. The layer structure is arranged within the body of the first test object, which makes it possible to keep the refractive index jumps at the interfaces from which the interference signal is obtained small. The OCT light propagates between the entrance surface and the layer structure within the entrance body, so that the refractive index jump at the first layer of the layer structure can also be kept small, thus avoiding a supersaturated interference signal. The entrance surface of the entrance body, which is shaped like a lens, allows the focus of the object beam path to be positioned appropriately.

[0011] The layer structure can be adapted to the axial resolution of the OCT device, which is often in the range of 1 µm to 50 µm. For example, the layer structure can comprise layers with a thickness between 20 µm and 500 µm, preferably between 50 µm and 200 µm. It is advantageous if the layer structure comprises several layers of the same thickness, i.e., if their thickness is as similar as can be achieved during manufacturing. The number of layers in the layer structure can, for example, be between 5 and 50, preferably between 10 and 30. Each of the layers can have the aforementioned characteristics.

[0012] Adjacent layers of the layered structure can consist of different materials. Layers that do not adjoin each other can consist of the same material. In one embodiment, the layered structure is composed of two different materials, with the layers alternating between the first and second materials.

[0013] To avoid oversaturation of the OCT signal, it is advantageous if the refractive index difference between adjacent layers of the layer structure is not too large. For example, the refractive index difference between the materials of two adjacent layers can be less than 0.1, preferably less than 0.075, and more preferably less than 0.05. The refractive index difference should not be below 0.001, because otherwise the acquired signal will no longer be sufficiently strong. This preferably applies to each layer interface of the layer structure. All values ​​for refractive index differences refer to the central wavelength of the OCT light.

[0014] Suitable transparent materials for the layered structure include, for example, glass materials, optical adhesives, epoxy resins, and silicones. One or more layers of the layered structure can consist of an optical adhesive with a group refractive index n. gOptical adhesives are transparent and characterized by a defined refractive index that lies within a specified range. It is also possible for one or more layers of the layer structure to consist of a silicone material with a group refractive index of n. g (840 nm) between 1.41 and 1.49. One or more layers of the layer structure can consist of a glass material with a group refractive index of n. g (840 nm) between 1.45 and 1.55, preferably between 1.46 and 1.55. One or more layers of the layer structure can consist of an epoxy resin with a group refractive index n g (840 nm) between 1.47 and 1.56. In one embodiment, one or more layers of the layer structure consist of borosilicate glass D263 M with n g(840 nm) = 1.516. A material is considered transparent if an OCT beam path can continue within the material. It is advantageous if the materials of the first test object are essentially crystal clear.

[0015] In one possible layered structure, layers of borosilicate glass alternate with layers of an optical adhesive. It is also possible for layers of borosilicate glass to alternate with layers of a silicone material. In an alternative embodiment, layers of borosilicate glass alternate with layers of epoxy resin. The layered structure can be created by using prefabricated glass components. These components can be placed into the other material while it is still liquid, so that after the other material hardens, a layered structure of glass and the other material is formed. If the prefabricated components are held at a suitable distance from each other even without the other material hardening, a layered structure is also possible in which the layers consist of a permanently liquid material, such as silicone oil.Intermediate layers between the glass and the other material should be avoided. This applies particularly to air inclusions between the glass and the other material.

[0016] The entrance element, positioned between the entry surface of the first test object and the first layer of the layer stack, can be made of a uniform material, ensuring that the entrance element is free of refractive index discontinuities. The entry surface of the first test object is defined as the area through which the OCT beam enters the material of the first test object. The refractive index difference between the material of the entrance element and a layer of the layer stack adjacent to the entrance element can be less than 0.1, preferably less than 0.075, and more preferably less than 0.05. Again, the refractive index difference should not be less than 0.001. The material of the entrance element can be one of the materials mentioned above as suitable for layers of the layer stack.In one embodiment, the material of the entry body is an optical adhesive, a silicone material, or an epoxy resin, and the material of the first layer of the layered structure is a glass material. During the fabrication of the first test object, the first layer of the layered structure can be embedded in the material of the entry body while the material of the entry body is still liquid. The first test object is preferably designed such that there are no inclusions of other materials, in particular no inclusions of air, between the entry surface and the layered structure.

[0017] Upon entering the body of the test object, a refractive index jump occurs, corresponding to the difference between the refractive index of air (approximately n=1) and the refractive index of the material of the entry object. If the entry object is made of one of the materials mentioned above, this results in a refractive index difference between 0.4 and 0.6. With such a high refractive index difference, the proportion of reflected OCT light is so high that the signal of a typical OCT device calibrated for smaller refractive index differences becomes oversaturated. Therefore, the procedure is preferably carried out such that the entry surface of the first test object lies outside the measurement window of the OCT device. The measurement window is defined as the axial region of the OCT beam path from which image information is acquired using OCT measurement. The measurement window can extend, for example, a few millimeters in front of and / or behind the reference plane.

[0018] The distance between the layer structure and the lens surface is preferably greater than 30%, more preferably greater than 50%, and more preferably greater than 80% of the axial length of the measurement window. In absolute terms, the distance between the layer structure and the lens surface is preferably greater than 3 mm, more preferably greater than 5 mm, and more preferably greater than 8 mm. All distances to the lens surface refer to the apex of the lens surface. If the layer structure has a thickness of 10 mm, this corresponds to an optical path length of approximately 15 mm. This is greater than the depth of typical OCT measurement windows, which usually ranges between 2 mm and 14 mm.

[0019] The propagation speed of OCT light within the body of the test object decreases according to the ratio of the refractive indices. Due to this reduced propagation speed, the reference plane for the OCT measurement within the test object is located closer to the entry surface than in an air-space measurement. In an air-space measurement, the OCT light propagates through air between the OCT device and the OCT measurement window.

[0020] Conversely, a structure detected within the test object by an OCT device calibrated for an air gap appears to be farther away than it actually is. For example, if a structure is 10 mm from the entry surface of the test object and the material of the test object has a group refractive index n g (840 nm) = 1.5, so in the OCT measurement the structure appears to have a distance of 15 mm to the entrance surface.

[0021] If the test object had a flat entry surface, the focus position of the OCT beam path would shift accordingly. In the example above, if an air-space measurement had a focus position 10 mm behind the position of the entry surface, the geometric focus position for a test object with a flat entry surface would be 15 mm behind the entry surface. Therefore, introducing a test object into the beam path of an OCT device calibrated for an air-space measurement generally leads to a shift between the reference plane of the OCT measurement and the focus position of the OCT beam path. The invention proposes to design the entry surface of the test object as a lens surface to counteract this shift in the focus position.

[0022] The lens surface can be shaped such that the distance between the focus position of the OCT beam path and the reference plane of the OCT measurement is reduced, preferably by at least 30%, more preferably by at least 50%, and more preferably by at least 70%, compared to a test object that has a flat entry surface instead of the lens surface. The lens surface can additionally or alternatively be shaped such that the difference between the geometric focus position in the first test object and the geometric focus position in the air gap is reduced, preferably by at least 30%, more preferably by at least 50%, and more preferably by at least 70%, compared to the case where a test object with a flat entry surface is introduced into the beam path instead of the first test object.In particular, the lens surface can be shaped such that the geometric focus position within the test object is the same as the focus position in the air gap, i.e., when no test object is positioned in the OCT beam path. For these comparisons, it is assumed that the flat entrance surface is positioned at the point on the lens surface closest to the OCT device. This is typically the apex of the lens surface.

[0023] If an OCT device is used to examine the retina rather than the anterior segment of the eye, the collimated OCT beam path strikes the cornea. Focusing the OCT beam path onto the retina is achieved by the optical elements in the anterior segment of the eye, specifically the cornea and the lens of the patient's eye. The first test object can also be used to test such an OCT device by positioning a lens in front of its lens surface. This lens shapes the collimated OCT beam path so that its focus lies within the layered structure. The refractive power of such a lens essentially corresponds to that of the cornea and lens of a patient's eye.

[0024] In one embodiment of the method according to the invention, a set of test objects is used, wherein the set comprises a first test object designed according to the invention and wherein the set comprises an alternatively designed second test object. The location of the OCT measurement window can be determined using the second test object by moving the second test object in the z-direction until the OCT device provides an image of a test surface of the second test object. This movement in the z-direction changes the axial distance between the second test object and the OCT device.

[0025] The second test object can have a non-reflective test surface that scatters incident OCT light. The test surface can be provided with a test structure having a known spatial distribution. The test structure can be designed as a three-dimensional structure of the test surface. In one embodiment, the test structure comprises a plurality of concentric circles.

[0026] Using a second test object with such a test structure, the size of the image field of an OCT measurement can be determined. This second test object can be positioned in the z-direction so that the OCT device generates an image of the test surface. With a circular test structure, the OCT beam path can be aligned perpendicular to the z-direction towards the center of the circular structure. Once an image of the test surface is generated by the OCT device, the size of the image field of the OCT measurement can be read by comparing the image with the actual test surface.

[0027] The method can be carried out by placing the second test object on a positioning surface and moving the positioning surface and the OCT device relative to each other to focus the OCT device on the test surface. Following an OCT measurement, the second test object can be removed from the positioning surface to allow a subsequent measurement with the first test object according to the invention.

[0028] If the distance between the OCT device and the positioning surface remains constant, the distance the lens surface must be from the positioning surface so that the focus of the OCT beam falls within the layer structure of the first test object can be determined based on the known parameters of the first test object. The first test object can have a support surface opposite the lens surface, with the distance between the support surface and the lens surface being dimensioned such that the layer structure is in a z-position that is visible in the OCT measurement, i.e., within the OCT measurement window. Preferably, the OCT device is positioned relative to the first test object such that the layer structure covers the OCT measurement window, i.e., that the layer structure covers the entire depth of the acquired OCT image. With a correctly aligned OCT device, the focus of the OCT beam path then also lies within the layer structure.The OCT device can be set so that the reference plane of the OCT measurement coincides with the near end of the OCT measurement window.

[0029] The first and second test objects can be coordinated such that the OCT beam path is focused on a test surface of the second test object when a support surface of the second test object is arranged at a predetermined axial distance to the OCT device, and that the OCT beam path is focused within the layer structure of the first test object when, instead of the second test object, the first test object is arranged in the OCT beam path and a support surface of the first test object has the same axial distance to the OCT device.

[0030] When an OCT measurement is performed on the first test object using such a setup, the resulting measurement signal has a peak at each of the interfaces of the layer structure. The depth of the OCT measurement window can be directly calculated from the number of peaks visible in the measurement and based on the known dimensions of the layer structure.

[0031] In a diagram where the amplitude of the measurement signal is plotted against the z-position, a curve can be generated that connects the maxima of the peaks. In OCT, the amplitude measurements are typically logarithmized. For example, if the signal S is measured by the detector, the amplitude A = 20*log(S) is stored. The invention proposes fitting the curve using the following equation. Amax(z)=10⋅log(I01+(z−z0Δz)2)+Aroll−off

[0032] In this equation, z is the variable that corresponds to the position of the peak maximum A. maxin the diagram. In this equation, the proportion A corresponds to roll-off the OCT roll-off. The other part of the equation represents the widening of the OCT beam with increasing distance from the OCT focus position z0.

[0033] The roll-off can be approximated by the following formula, which decreases towards the end of the OCT measurement window: Aroll−off=sin((z−zroll)⋅a)(z−zroll)⋅a⋅b

[0034] Alternatively, in a simplifying assumption, it can also be assumed that the roll-off has a linear progression that can be fitted with a*z+b.

[0035] Δz is the Rayleigh length of the OCT beam. The Rayleigh length Δz corresponds to the distance in the z-direction over which the OCT beam, starting from the focus position z0, doubles its cross-sectional area. The focus position z0 corresponds to the z-position at which the OCT beam path has the smallest cross-section, i.e., the z-position of the smallest constriction of the OCT beam. The constants I0 and b are required to determine the course of the peak maxima A. max (z) to be able to fit the equation. The relevant quantities in the equation, which are related to properties of the OCT device, are the OCT focus position z0, the Rayleigh length Δz, and the OCT roll-off constant z. roll and a.

[0036] The OCT roll-off constants z rollThe values ​​and are independent of the optics of the first test object and can therefore be directly applied to air distance measurements. In contrast, the determined values ​​for the OCT focus position z0 and the Rayleigh length Δz refer to the conditions within the first test object. Applying these values ​​to air distance measurements requires conversion based on the known optical parameters of the first test object.

[0037] Compared to alternative methods where the relevant parameters of the OCT device are determined directly by measuring the air distance, the method according to the invention offers the advantage of improved resolution. This results from the fact that the refractive properties of the first test object lead to a smaller Rayleigh length Δz and thus a higher numerical aperture of the OCT beam path. A further advantage lies in the insensitivity of the method according to the invention to alignment issues. It has no adverse effects on the measurement if the test object is not precisely aligned with the optical axis, but is, for example, tilted by a few degrees. This differs from alternative methods where the OCT beam path is reflected off a mirror to test the OCT device.

[0038] An interface between two layers of the layer structure has no axial extent, therefore the axial resolution of the OCT device can be determined from the width of a peak generated at such an interface. It should be noted that, due to group velocity dispersion (GVD) in the material of the first test object, the peaks are wider than in an air gap measurement. The conversion to the full width at half maximum (FWHM) for a peak in an air gap measurement is performed using the following formula. δzph=δzair2+(c⋅GVD(λc)⋅d⋅Δλ)2

[0039] In this formula, δz air the axial resolution of the OCT device in air, which ideally corresponds to the coherence length l C This corresponds to the OCT light source. The parameter d represents the distance over which the beam propagates within the material of the first test object. λ Cis the central wavelength of the OCT light and Δλ is the spectral width of the OCT light source. The wavelength-dependent group velocity dispersion GVD(A) of the material within which the OCT light propagates is known. The width δz ph The peak resolution, which corresponds to the axial resolution in the test specimen, is read from the measured data. Resolved for δz air (Dissolution into thin air) the relationship is as follows. δzair=δzph2+(c⋅GVD(λc)⋅d⋅Δλ)2

[0040] The relevant parameters for an air distance measurement can therefore be derived from the measurement according to the invention.

[0041] Furthermore, measurements taken with the first test object can provide clues about the image field curvature. For this, it is necessary to know the image field size. It must also be taken into account that, in the measurement according to the invention, image field curvature is also caused by the material of the first test object. The portion of the image field curvature caused by the first test object must be subtracted in order to be able to draw conclusions about the image field curvature in a clearance measurement.

[0042] It is also possible to derive information about the numerical aperture of the OCT device from measurements taken with the first test object. For this purpose, the scan range of the device can be determined. The scan range is defined as the area around the optical axis within which a signal can still be acquired by the OCT device. When scanning laterally, i.e., shifting the OCT beam path perpendicular to the optical axis, the signal strength decreases with increasing distance from the optical axis. The signal disappears when the end of the scan range is reached. The distance between the optical axis and this end of the area accessible by measurement corresponds to half the scan range. It has been found that there is a linear relationship between the easily determined scan range and the numerical aperture.The numerical aperture of the OCT device can therefore be determined based on a functional relationship between the scan width of the OCT measurement and the numerical aperture of the OCT device. The method according to the invention thus offers a simple way to determine a characteristic parameter of the OCT device that would otherwise only be ascertainable with considerable effort.

[0043] It is also possible to derive information about the OCT sensitivity from measurements taken with the first test object. OCT sensitivity describes the smallest possible detectable OCT signal and is therefore an important parameter for describing the quality of an OCT device. The amplitude in the OCT image behaves according to the following formula: A(z)≈20⋅log(Iref⋅I(z)sample)

[0044] This is I ref the intensity of the OCT reference light and I(z) Probethe light intensity that returns to the device from the respective depth of the test specimen.

[0045] From an OCT measurement without a test object, the depth-dependent noise signal A can be determined. Rauschen can be determined. The maximum amplitude A can be determined from the OCT measurement of the test object. max,fokus The signal-to-noise ratio of the OCT measurement of the test object is determined in the focal plane. This yields the signal-to-noise ratio of the OCT measurement of the test object with: SNR=(Amax,focus−ARouscea)⋅F

[0046] The amplitude A max,fokus The amplitude in the test specimen is reduced by a factor F compared to the amplitude without group dispersion velocity (i.e., a measurement in air). This factor can be determined. For example, SNR = 53.2 dB. The reflectivity of a layer transition in the test specimen is also known and is calculated using the following formula: R(zfocus)=(n1−n2n1+n2)2=I(zfocus)sampleI0,sample

[0047] Here, n1 is the refractive index of the first layer material and n2 is the refractive index of the second layer material. 0,Probe is the light intensity and I(z) incident on the test specimen fokus ) Probe This is the light intensity that returns from the focal plane of the sample to the OCT device. For the sample described here, the reflectivity of a layer interface is, for example, R = 0.00002. This corresponds to an OCT sample light intensity of 10*log(R) = -47 dB. Conceptually, one reduces the maximum amplitude measured in the sample at the focal plane by 10*log(R) to obtain the sensitivity of the OCT device. Sensitivity=SNR−10⋅log(R(zfocus))

[0048] In our example, that's 53.2 dB - (-47 dB) = 100.2 dB.

[0049] The invention also relates to a test object intended for use in testing an OCT device. The first test object comprises a layered structure consisting of a plurality of layers and an entrance body. The layered structure and the entrance body are made of materials transparent to OCT light. An entrance surface of the entrance body is shaped as a lens surface. Such a test object can be used as the first test object in the process according to the invention.

[0050] The disclosure includes further developments of the test object with features that are described in connection with the method according to the invention.

[0051] The invention is described below by way of example with reference to the accompanying drawings and advantageous embodiments. The drawings show: Fig. 1: A schematic representation of an OCT device in use on a human eye; Fig. 2, Fig. 3: Details of the beam path of the OCT device from Fig. 1; Fig. 4: the OCT device from Fig. 1 during a measurement on a second test object; Fig. 5: the OCT device Fig. 1 during a measurement on a first test object; Fig. 6: an OCT image taken of the second test object; Fig. 7: a sectional view of a first test object; Fig. 8: an OCT image taken of the first test object; Fig. 9: one aspect of the OCT image from Fig. 8 in another representation; Fig. 10: one of an excerpt from Fig. 9 outgoing evaluation; Fig. 11: two different positions of the first test object in the OCT beam path; Fig. 12: a representation of the linear relationship between the scan width of the measurement and the numerical aperture of the OCT device.

[0052] A in Fig. The OCT device 14 shown has an exit aperture 15 through which an OCT beam path 16 generated in the OCT device is directed as an object beam path onto a human eye 17. Scattered portions of the OCT light are reflected back into the OCT device 14 by the transparent structures in the anterior segment of the eye 17 and caused to interfere with a reference beam path. The position of scattering centers in the eye 17 can be determined from the interference signal. A scanning device (not shown) is integrated into the housing of the OCT device 14, which deflects the OCT beam path 16 laterally so that the OCT beam path 16 scans the eye 17. The measurement data acquired from a single position of the OCT beam path 16 are referred to as an A-scan. From a large number of A-scans, a cross-sectional image of eye 17 (B-scan) or a three-dimensional image of eye 17 can be generated.

[0053] The OCT beam path 16 has according to Fig. 2. A central ray 18 extends in the z-direction. With respect to the central ray 18, the OCT beam path 16 has a radial extent, exhibiting a minimum constriction 19 at position z0. Position z0 corresponds to the focus of the OCT beam path 16. As the distance in the z-direction from the minimum constriction 19 increases, the cross-section of the OCT beam path 16 widens. The distance Δz, within which the cross-sectional area doubles from the minimum constriction 19, is called the Rayleigh length 20. Since the resolution of an OCT measurement decreases with increasing distance from the minimum constriction 19, the Rayleigh length 20 is a relevant parameter for the operation of an OCT device 14.

[0054] The OCT beam path 16 enters the transparent structures of the eye 17, which according to Fig. 3. The cornea 22 and the lens 21 are included. The focus position 24 at position z0 of the OCT beam path 16 lies within the transparent structures 21, 22. The reference plane 23 of the OCT measurement, in which the OCT beam path 16 (= object beam path) and the reference beam path have the same length, is located in front of the cornea 22. The measurement window of an OCT measurement is the axial section, i.e., the section in the z-direction, from which measurement data are obtained using the OCT measurement. The measurement window can, for example, include an area extending from the anterior surface of the cornea 22 to behind the lens 21.

[0055] After a period of use in examinations of patients' eyes, the OCT device 14 will undergo a check. This check will determine, among other things, whether the OCT device 14 is correctly focused and whether the desired axial resolution is achieved across the measurement window.

[0056] In a first step of the inspection, the OCT beam path 16 is directed at a second test object 26, see Fig. 4. The second test object 26 rests with a support surface 31 on a measuring table 25, the surface of which forms a positioning surface 28. For the OCT image, the OCT device 14 is adjusted so that the focus position 24 of the OCT beam path 16 coincides with the top surface of the second test object 26.

[0057] The top surface of the second test object 26 has a three-dimensional test structure in the form of concentric circles. The second test object 26 is made of an opaque material, on whose surface the OCT light is scattered. For measurement, the second test object 26 is positioned laterally, i.e., perpendicular to the z-direction, on the measuring table 25 such that the OCT beam path 16 intersects the center of the test structure. During the measurement, the OCT beam is deflected laterally by a scanning device located in the housing of the OCT device 14, so that the OCT beam scans the surface of the second test object 26 in a single scan.

[0058] An OCT image of the second test object 26, generated in this way, is in Fig. Figure 6 shows a rectangular section of the surface, in the center of which the center point of the test structure is located. The rings on the surface of the second test object 26 have a spacing of 200 µm. From the number of in Fig. The 6 visible rings indicate that the OCT device 14 is set to a lateral scan width of 4 mm x 1 mm.

[0059] In a second step of the test, the second test object 26 is removed from the measuring table 25 and a first test object 27 is positioned with its support surface 34 on the measuring table 25. The distance between the OCT device 14 and the measuring table 25 remains unchanged.

[0060] The first test object 27 comprises a layer structure 29 and an entrance body 30. The layer structure 29 consists of a total of twenty layers, alternating between borosilicate glass D263M with n(840nm) = 1.516 and an optical adhesive NOA76 with n(840nm) = 1.504. The entrance body 30, which is directly adjacent to the foremost layer of the layer structure 29 made of borosilicate glass, also consists of the optical adhesive NOA76 with n(840nm) = 1.504. Therefore, after the OCT beam path 16 has entered the body of the first test object 27, there are no longer any refractive index discontinuities greater than 0.02. With such small refractive index jumps, only a small part of the OCT light is reflected back towards the OCT device 14, so that the intensity of the reflected OCT light is no greater than that of the transparent structures in the anterior segment of a patient's eye 17. Oversaturation of the OCT signal is avoided.

[0061] The entry surface of the entry body 30, through which the OCT beam path 16 enters the body of the first test object 27, is shaped as a lens surface 31, thereby bringing the focus position 24 of the OCT beam path 16 closer to the OCT device 14. The first test object 27 is designed such that the layer structure 29 is located at a distance in the z-direction from the positioning surface 28 of the measuring table 25. This distance is dimensioned such that the layer structure 29 lies within the OCT measurement window of the OCT device. With a correctly configured OCT device 14, the focus position 24 of the OCT beam path 16 is then automatically located within the layer structure 29. Preferably, the measurement on the first test object 27 is performed such that the focus position 24 lies approximately in the center within the layer structure 29. The reference plane of the OCT measurement can coincide with the near end of the OCT measurement window.If the first test object 27 were made of air, its surface would have to be shifted to plane 23 so that the layer structure 29 is visible in the OCT measurement window. The plane 23 of the OCT measurement lies approximately halfway between the layer structure 29 and the apex of the lens surface 31. According to . Fig. 5 in the test object 27 the layer structure 29 and the entry body 30 are arranged within a housing 32, the underside of which forms the support surface 34 of the first test object 27.

[0062] In Fig. Figure 8 shows an OCT image in the form of a B-scan acquired on the first test object 27. The B-scan is obtained by scanning the first test object 27 once in a transverse direction with the OCT beam, for example in Fig. 7 within the plane of the figure. For each position of the OCT beam, an A-scan is acquired, from which image information is obtained pointing in the z-direction into the depth of the first test object 27. The B-scan is composed from a plurality of A-scans.

[0063] Each interface between two layers of the layer structure 29 generates a peak of the interference signal, see the areas shown in light in Fig. 8. The depth of the OCT measurement window can be read by counting the number of interfaces visible in the B-scan.

[0064] The actually flat interfaces of the first test object 27 appear to be in Fig. 8 to have a slight curvature. The image field curvature results from the fact that the OCT light propagates over a longer distance within the optical adhesive material of the entry body 30 before the layer structure 29. In a corresponding air gap measurement, in which the OCT light propagates in air to the beginning of the measurement window, the OCT device 14, with which the OCT image is taken in Fig. 8, which was generated, shows no field curvature. Does an OCT image acquired on the first test object 27 have a different field curvature than in Fig. 8, so the presence of image field curvature in air distance measurements can be inferred.

[0065] The interference signal shows a peak at each of the interfaces of the layer structure 29. Fig. Figure 9 shows the peaks of a single A-scan, with the horizontal axis corresponding to the z-direction. The amplitude of the peaks decreases from a maximum in two directions. This decrease is due to two superimposed effects: the increasing distance from the focus position 24 of the OCT beam path 16 and the increasing distance from the reference plane 23 of the OCT measurement. The decreasing amplitude with increasing distance from the OCT reference plane 23 is called the OCT roll-off.

[0066] Against this background, the amplitude profile can be approximated with the following equation, which takes into account both the distance from the focus position and the OCT roll-off. Amax(z)=10⋅log(I01+(z−z0Δz)2)+sin((z−zroll)⋅a)(z−zroll)⋅a⋅b

[0067] Here, z is the variable that corresponds to the position of the peak maximum A. max in Fig. 9 corresponds to the sinc function. The sinc function corresponds to the OCT roll-off. The other part of the equation represents the widening of the OCT beam with increasing distance from the focus position 24. Δz is the Rayleigh length 20 of the OCT beam. The constants I0 and b are needed to determine the shape of the peak maxima A. max (z) to be able to fit using the equation. The quantities relevant for testing the OCT device are the OCT focus position z0, the Rayleigh length Δz, and the OCT roll-off constant z. roll and a.

[0068] From the measurement in Fig. 9 results in the OCT roll-off function: Aroll−off=sin((z−zroll)⋅a)(z−zroll)⋅a⋅b It is advantageous here to subtract the value at 90% of the OCT measurement window depth from the value at 10% of the OCT measurement window depth A. roll-off (10%z max ) -A roll-off (90% max ) to determine. This value illustrates how much the OCT signal decreases with measurement depth.

[0069] For example, focus position 24 is located at position z0, which is 0.7 mm below the upper end of the measuring window. For the Rayleigh length 20, a value of Δz = 0.58 mm is determined here. This results in a numerical aperture of 0.81 and a lateral resolution of 6.34 µm in the first test object.

[0070] The OCT roll-off is independent of the optics of the first test object 27 and therefore also applies to an air gap measurement at the patient's eye, where the OCT light propagates along an air gap between the OCT device and the measurement window. In contrast, the determined values ​​for the OCT focus position z0 and the Rayleigh length Δz refer to the conditions within the first test object. Transferring these values ​​to the air gap requires a conversion based on the known optical parameters of the first test object.

[0071] For the in Fig. In the measurement shown in Figure 9, OCT light with a central wavelength of λ = 840 nm was used. The material of the entrance body 30 has a refractive index n(840 nm) = 1.50 and a group refractive index n g (840nm) = 1.51. Based on the information obtained from the measurement and the known geometric parameters of the first test object 27, and applying Snell's law of refraction, the focus position and the Rayleigh length 20 for an air distance measurement can be calculated.

[0072] For the air distance measurement, a numerical aperture of 0.036 and a focus position of z are obtained. 0,Luft of 1.26 mm below the upper end of the measuring window and a lateral resolution of 14.36 µm.

[0073] In Fig. 10 is one of the peaks from Fig. 9 is shown with higher resolution in the z-direction. The wider peak 35, whose mathematical approximation is marked with the triangles, corresponds to the measured values ​​from Fig. 9. The broadening of the peak results from the group velocity dispersion to which the broadband OCT light is subject within the material of the entrance body 30. If the group velocity dispersion is subtracted, a narrower peak 36 is obtained, which corresponds to the axial resolution in an air gap measurement.

[0074] In Fig.Figure 11 shows two different positions that the first test object 27 can have relative to the optical axis 37 of the OCT beam path 16. The test object 27 shown on the left is aligned with the optical axis 37, such that the optical axis 37 extends centrally through the first test object 27 and intersects the layer structure 29 at a right angle. The test object 27 shown on the right is tilted relative to the optical axis 37, so that the central axis of the first test object 27 forms an angle with the optical axis 37 that is significantly different from 0°. For both positions, a partial OCT beam path 16 is indicated, which is shifted in the scan direction 38 relative to the optical axis 37. The illustration demonstrates that the tilting of the first test object does not negatively affect the measurement result.This insensitivity to adjustment is an advantage over alternative methods in which the OCT beam path 16 is reflected off a simple mirror surface.

[0075] In the course of applying the method according to the invention, it has been found that there is a linear relationship between the scan distance W and the numerical aperture NA of the OCT device 14. The scan distance W indicates how far the OCT beam path 16 can be shifted in the scan direction 38 without the OCT signal disappearing. The scan distance W is a quantity that can be easily determined. It is an advantage of the method according to the invention that this linear relationship provides a simple way to determine the numerical aperture NA of the OCT device 14.

Claims

[1] Method for testing an OCT device (14) in which the axial resolution, the axial depth of the measurement window, the OCT roll-off, the numerical aperture, the OCT sensitivity, the signal-to-noise ratio and / or the image field curvature of the OCT device (14) are determined, in which a first test object (27) is arranged in an OCT beam path (16) of the OCT device (14), wherein the first test object (27) comprises a layer structure (29) of a plurality of transparent layers and an entrance body (30), wherein OCT light emitted from the OCT device (14) enters the entrance body (30) via an entrance surface and propagates through the entrance body (30) to the layer structure (29), and wherein the entrance surface of the entrance body (30) is shaped as a lens surface (31), and wherein by the entrance surface shaped as a lens surface (31) is positioned as the focus of the OCT beam path (16) within the layer structure (29). [2] Method according to claim 1, wherein the layer structure (29) comprises layers whose thickness is between 20 µm and 500 µm, preferably between 50 µm and 200 µm. [3] Method according to claim 1 or 2, wherein the layers of the layer structure (29) alternately consist of a first material and a second material. [4] Method according to any one of claims 1 to 3, wherein the refractive index difference between the materials of two adjacent layers of the layer structure (29) is less than 0.1, preferably less than 0.075, more preferably less than 0.

05. [5] Method according to any one of claims 1 to 4, wherein the refractive index difference between the material of the entry body (30) and a layer of the layer structure (29) adjacent to the entry body is less than 0.1, preferably less than 0.075, further preferably less than 0.

05. [6] Method according to any one of claims 1 to 5, wherein the entry surface of the entry body (30) is located outside the measuring window of the OCT device (14). [7] Method according to any one of claims 1 to 6, wherein the lens surface (31) is shaped such that the distance between a focus position (24) of the OCT beam path (16) and a reference plane (23) of the OCT measurement is reduced by at least 30%, further preferably by at least 50%, further preferably by at least 70% compared with a test object which has a flat entry surface instead of the lens surface (31). [8] Method according to any one of claims 1 to 7, wherein the size of the image field of the OCT measurement is determined using a test structure of a second test object (26). [9] Method according to claim 8, wherein the first test object (27) and the second test object (26) are aligned such that the OCT beam path (16) is focused on a test surface of the second test object (26) when a support surface (33) of the second test object (26) is arranged at a predetermined axial distance to the OCT device (14), and that the OCT beam path (16) is focused within the layer structure (29) of the first test object (27) when the first test object (27) is arranged in the OCT beam path (16) instead of the second test object (26) and a support surface (34) of the first test object has the same axial distance to the OCT device (14). [10] Method according to any one of claims 1 to 9, wherein the numerical aperture (NA) of the OCT device (14) is determined based on a functional relationship between the scan width (W) of the OCT measurement and the numerical aperture (NA) of the OCT device (14). [11] Method according to any one of claims 1 to 10, wherein information about the OCT sensitivity is obtained by means of a measurement with the first test object (14). [12] Test object for use in the testing of an OCT device (14) by means of a method according to one of claims 1 to 11, comprising a layer structure (29) of a plurality of layers and an entrance body (30), wherein the layer structure (29) and the entrance body (31) consist of materials transparent to OCT light and wherein an entrance surface of the entrance body is shaped as a lens surface (31).

Images