Interferometric measuring device

Abstract

Interferometric measuring device (1) for measuring a surface (20, 30) or a profile of an optical element (10) having two opposite surfaces (20, 30) and a measuring method comprising the steps of: - defining at least three measuring points (21, 31, 22, 32, 23, 33) on one of the surfaces (11) of the optical element (10), - measuring the position of each measuring point by directing a measuring light beam (61) from a measuring head (60) onto the measuring point and by detecting the reflected measuring light beam portion (62) at the measuring point (21, 31), respectively, - determining at least one of a decentration (D) and a tilt (T) of the measuring surface (11) with respect to a reference axis (51) based on the at least three measuring positions.

Description

Technical Field

[0001] This disclosure relates to the field of interferometric measurement apparatuses, and more particularly to interferometric measurement apparatuses implemented in optical fibers for measuring the surface, distance, and / or profile of an object by means of reflection of electromagnetic radiation from the surface of the object. Background Technology

[0002] Quality control and precision measurement of optical surfaces are becoming increasingly important for optical components such as lenses, especially for mass-produced optical components. This is particularly relevant for very small-sized optical components used in, for example, small camera systems, imaging systems, and / or display systems.

[0003] For example, patent DE 10 2011 011 065 B4 discloses an apparatus for measuring at least one surface segment of an object mounted on a carrier. The apparatus includes: a reference object fixed relative to the carrier; and a support movable relative to the reference object in at least a first direction.

[0004] A reference body and a distance sensor (rotatably mounted relative to each other) are arranged on a bracket. A distance measuring device is configured to determine a first distance from a first point on the surface of an object, and a second distance from a second point on the reference body corresponding to the first point on the object. For this purpose, the distance measuring device includes a first distance sensor and a second distance sensor, one facing the object and the other facing the reference body. Using such a device, the surface of an object can be optically detected or scanned with high precision and in a non-contact manner.

[0005] To measure thickness, particularly to measure the thickness profile of an object, and to determine the wedge shape and tilt of an optical surface, document US 2017 / 0082521 A1 discloses another apparatus utilizing an object holder having upper and lower sides, wherein the object holder is selectively adapted to be arranged on a carrier in a first orientation and a second orientation. To measure the thickness of the object and to mutually assign surface profiles disposed on opposite sides of the object, it is required to measure or scan the first or upper surface of the object when the object holder is positioned in the first orientation. Subsequently, when the object holder is in the second orientation, it is necessary to measure or scan the object.

[0006] The placement of the object support relative to the distance measuring device and the orientation of the object support on which the object is mounted may require manual adjustment and / or calibration of the corresponding measuring device, which can sometimes be very complex and time-consuming.

[0007] Such calibration and adjustment may be particularly relevant, for example, when the object to be measured and a corresponding quantity of the object are placed on a rotating measuring stage, which rotates during the scanning of the surface of the optical element or object. For such a measurement scheme, it is generally required that the optical axis of the optical element to be measured be aligned with the rotation axis of the measuring stage.

[0008] Especially for surface or distance measurements based on interferometry, the distance between the measuring head and the surface of the optical element being measured must be kept within a predefined measurement range. For optical interferometry-based measurement methods, the wavelength of the measuring beam determines the unambiguous measurement range, or region, corresponding to the distance measurement. The unambiguous measurement range can be expanded, for example, by simultaneously using multiple measuring beams of different wavelengths. However, this generally follows the rule that an increase in the spatial resolution of the measurement is accompanied by a corresponding decrease in the unambiguous measurement range.

[0009] Therefore, the purpose of this disclosure is to provide an improved method for measuring surfaces, and an improved measuring apparatus, such as an interferometric measuring apparatus for measuring the surface or profile of an optical element. The measuring apparatus and method should provide highly efficient and rapid measurements, as well as calibration and adjustment of the interferometric measuring apparatus. Furthermore, the method and measuring apparatus should provide accurate and rapid measurements of the eccentricity, thickness, and / or tilt of at least one surface or opposing surface of the optical element without requiring modification of the support or mounting of the optical element relative to the measuring apparatus.

[0010] Another objective of this disclosure is to provide corresponding computer programs for performing such measurement methods and controlling the corresponding interferometric measuring devices. Summary of the Invention

[0011] In one aspect, a method for measuring the surface and / or profile of an optical element is provided. The optical element includes a first surface and a second surface. The first and second surfaces are disposed on opposite sides of the optical element. The first surface may be the top surface of the optical element, and the second surface may be the bottom surface of the optical element. The optical element may be any type of optical lens.

[0012] In the first step, the method includes defining at least a first measurement point, a second measurement point, and a third measurement point on the measurement surface of the optical element. The measurement surface is one of a first surface and a second surface. The measurement points are predefined measurement points. They can be selected and / or defined according to a variety of criteria. For the method of measuring the surface or profile of an optical element according to the present invention, the corresponding first measurement point, second measurement point, and third measurement point are defined and, at least during the measurement of the surface or profile of the optical element, are at least substantially fixed on the measurement surface.

[0013] Typically, the shape and / or profile of an optical element are known with at least minimal precision. Within this range, the surfaces and / or profiles of the optical element are known in quantity. For the method of the present invention, the surfaces and / or profiles need to be measured precisely with a maximum precision (which is certainly higher than the known minimum precision). The minimum precision can be provided by the manufacturer of the optical element. The maximum precision can reveal manufacturing tolerances and / or defects in the surface or profile of the optical element. For typical embodiments, the minimum precision is in the micrometer range. The maximum precision can be in the range of only a few nanometers, for example, less than 50 nm, less than 20 nm, or less than 10 nm.

[0014] The method of the present invention is used to provide quality control, and the method of the present invention is configured to measure the surface and / or profile of an optical element with maximum accuracy.

[0015] After defining at least a first measurement point, a second measurement point, and a third measurement point on the measurement surface, the first position of the first measurement point is measured by guiding a measurement beam from the measurement head to the first measurement point. A portion of the measurement beam reflected (typically retroreflected) at the first measurement point is detected. By detecting the reflected measurement beam portion, the position of the first measurement point can be determined or calculated. To measure the first position of the first measurement point on the measurement surface, a non-contact optical measurement procedure is typically applied. The measurement procedure may include runtime analysis and / or analysis of the relative phase between the reflected measurement beam portion and a reference beam.

[0016] Typically, the measuring head is configured to guide a measuring beam to a first measuring point and detect the portion of the measuring beam reflected at that point. Typically, the measuring head is aligned along the surface normal of the measuring surface near or in the region at the first measuring point. Typically, the measuring beam is guided perpendicularly onto the measuring surface at the first measuring point. Therefore, the portion of the measuring beam reflected at the first measuring point re-enters the measuring head.

[0017] The deviation of the orientation of the measuring beam from the surface normal of the first measuring point is acceptable for the predefined angle of acceptance of the measuring head. Typically, the deviation of the acceptance angle from the surface normal of the first measuring point is no greater than 15°, 10°, or 5°.

[0018] Subsequently, at least the second position of the second measurement point and the third position of the third measurement point are measured successively. Measurements of the corresponding first and second measurement points are performed in the same manner as described above for measuring the first measurement point. Here, the measurement beam is guided to the second measurement point, and typically the corresponding portion of the measurement beam reflected at the second measurement point is detected by the measurement head.

[0019] In the same manner, the measurement beam is guided to at least the third measurement point, and typically the corresponding portion of the measurement beam reflected at the third measurement point is detected by the measurement head.

[0020] Based on at least a first position, at least a second position, and at least a third position, determine at least one of the eccentricity and tilt of the measuring surface relative to the reference axis.

[0021] The reference axis can be defined by the measuring stage of the measuring device, and the optical element can be positioned on the measuring stage for surface and / or profile measurement. The reference axis can be a rotation axis about which the optical element can rotate during a surface scan or profile scan to determine at least one of eccentricity and tilt of the measured surface.

[0022] In principle, at least the first position, at least the second position, and at least the third position can be compared with the corresponding reference position. If the optical element is aligned with the reference axis (e.g., with zero tilt and / or zero eccentricity), then at least the first measurement point, the second measurement point, and the third measurement point will be positioned in the corresponding reference position.

[0023] Typically, the first, second, and third positions of the actual measured points can be numerically analyzed and fitted to a mathematical model that takes the eccentricity and tilt of the measured surface as variables. Through numerical calculations (e.g., by fitting the first, second, and third positions to corresponding first, second, and third reference positions), the eccentricity and / or tilt of the measured surface relative to a reference axis or relative to a corresponding reference surface (e.g., a reference surface defined by the first, second, and third reference points) can be determined.

[0024] Typically, the measured surface or the measured profile can be derived based on at least a first measurement point, a second measurement point, and a third measurement point. The measured surface or profile can be compared with, or fitted to, the mathematical model of the corresponding reference surface or profile.

[0025] In this way, and simply by measuring the first, second, and third positions on the measuring surface, the eccentricity and tilt of the measuring surface can be determined qualitatively and / or quantitatively.

[0026] Typically, the number of measurement points is limited by the type of measurement surface. Generally, the first, second, and third measurement points must not be on the same straight line. A line can be drawn through the first and second measurement points; however, the third measurement point must be located off-line. If the measurement surface is spherical, it can be definitively characterized by the first, second, and third measurement points.

[0027] When the measurement surface is non-spherical, at least five measurement points must be defined and measured. When the optical element includes a so-called free-form surface as the measurement surface, at least six measurement points will be required. These at least six measurement points must be defined and subsequently measured using the methods described above to explicitly determine or calculate the eccentricity and / or tilt of the measurement surface. The number of measurement points depends on the number of degrees of freedom required to define the corresponding measurement surface.

[0028] In a typical embodiment, the eccentricity and tilt determined by the method described above can be directly used to adjust and / or align the optical element for subsequent high-precision surface or profile measurements using the same measuring device. In this way, the method described above for determining at least one of the eccentricity and tilt of the measuring surface of the optical element can be directly used to align the optical element relative to the measuring head of the measuring device.

[0029] By determining at least one of eccentricity and tilt, the method can provide quantitative feedback on the direction and degree of eccentricity and tilt. By precisely determining at least one of the eccentricity and tilt of the measurement surface, even fully automatic or semi-automatic adjustment or calibration of optical elements relative to the measuring head or relative to a reference axis can be provided. This allows for a reduction in the expenditure and effort required for precisely aligning and calibrating optical elements for performing high-precision surface and / or profile measurements using measuring devices (e.g., interferometric measuring devices).

[0030] According to another embodiment, measuring at least one of the first position, second position, and third position includes focusing a measurement beam onto at least one of the first measurement point, second measurement point, and third measurement point, and detecting the corresponding portion of the measurement beam reflected backward at at least one of the first measurement point, second measurement point, and third measurement point. By focusing the measurement beam onto the first measurement point, the receiving angle of the measuring head for the reflected portion of the measurement beam can be increased.

[0031] Furthermore, focusing the measurement beam onto the corresponding measurement point increases the amount of light reflected at that point. This increases the signal-to-noise ratio, which is useful for detecting the portion of the measurement beam that reflects the light.

[0032] According to a further embodiment, one of the first and second surfaces of the optical element faces the measuring head. The other of the first and second surfaces faces away from the measuring head. This other surface may face the measuring stage, on which the optical element is mechanically supported. Here, the second surface facing away from the measuring head and potentially facing the measuring stage is the measuring surface. The first, second, and third positions are measured by guiding a measuring beam to a first target point positioned on one of the first and second surfaces facing the measuring head. Here, the measuring beam propagates through the medium of the optical element and strikes the first measuring point. The measuring beam is reflected at least partially at or near the first measuring point. Typically, it is reflected backward through the medium toward the first target point and propagates back from the first target point to the measuring head.

[0033] In the same manner, the method further continues to measure the positions of the second and third measurement points through the medium of the optical element. To measure the position of the second measurement point, the measurement beam is guided to a second target point on one of the first and second surfaces facing the measurement head. Similarly, to measure the position of the third measurement point, the measurement beam is guided to a third target point, with one of the first and second surfaces facing the measurement head located at the third target point.

[0034] The first target location is directly related to the first measurement point. The second target point is directly related to the second measurement point, and the third target point is directly related to the third measurement point. If more than three measurement points are set, a corresponding number of target points will also be set on the surface opposite to the measurement surface.

[0035] Typically, the measurement point is set on the common surface of the first and second surfaces. Furthermore, the target point is set on the common surface of the first and second surfaces of the optical element. The target point is always set on the surface opposite to the surface where the measurement point is located.

[0036] By guiding the measuring beam to at least a first target point, a second target point, and a third target point (e.g., on a first surface), the position of a measuring point on a second surface can be determined by measurement. This has the advantage that measuring points positioned on a measuring surface facing away from the measuring head can be measured directly without having to reorient the optical elements on the measuring stage (e.g., toward the measuring head).

[0037] The method of measuring the surface and / or profile of an optical element according to the present invention can be used to measure surfaces facing the measuring head as well as surfaces facing away from the measuring head. In this way, measurements can be performed through a medium passing through the optical element. This subsequently allows for the measurement of the eccentricity and / or tilt of the first and second surfaces relative to a reference axis. Such subsequent measurements can be performed in a common coordinate system or reference system. Therefore, the tilt and / or eccentricity of the first surface can be directly correlated with the tilt and / or eccentricity of the second surface.

[0038] Therefore, in the first measurement procedure, the eccentricity and tilt of the first surface of the optical element can be measured by measuring at least the first, second, and third positions of a first measurement point, a second measurement point, and a third measurement point positioned on the first surface of the optical element. Thereafter, the measurement surface can be switched to the second surface without moving the optical element relative to the measurement stage. During the second measurement procedure, at least one of the eccentricity and tilt of the second surface can be determined by subsequently guiding a measurement beam to at least the first, second, and third target points on the first surface and detecting the corresponding reflected beam portions from the first, second, and third measurement points on the second surface. In this way, at least one of the eccentricity and tilt of the second surface of the optical element can be determined.

[0039] Typically, the eccentricity and tilt of the first measuring surface can be determined relative to a reference axis. Since the optical element is not moved relative to the measuring stage during subsequent measurement procedures on the first and second surfaces, at least one of the eccentricity and tilt of the second surface of the optical element can be determined relative to the same reference axis. In this way, at least one of the eccentricity and tilt of the first surface of the optical element can be directly correlated with at least one of the eccentricity and tilt of the second surface of the optical element, respectively.

[0040] For the mutual mapping or assignment of the eccentricity and / or tilt of the first surface relative to at least one of the eccentricity and tilt of the second surface, the optical element can be kept stationary on the measuring stage. In this way, it is unnecessary to reorient the optical element between the first measuring procedure (in which the first surface is the measuring surface) and the second measuring procedure (in which the second surface is the measuring surface).

[0041] According to a further embodiment, the position of one of the first and second surfaces facing the measuring head at the first target point, the second target point, and the third target point is determined based on at least one of the following factors: the refractive index of the medium of the optical element; the incident angle of the measuring beam at the first target point, the second target point, and the third target point; and the local surface profile of at least one of the first and second surfaces in the region of the first, second, and third measuring points and / or in the region of the first, second, and third target points.

[0042] Furthermore, the mapping or assignment between the first target point and the first measurement point is also determined based on the thickness or thickness distribution of the optical element. It may also depend on the distance between the first and second surfaces in the region between the first measurement point and the first target point. This is equally valid for the mapping between the second measurement point and the second target point, as well as for the mapping and assignment between the third measurement point and the third target point.

[0043] Typically, at least a first, second, and third measurement point are defined on a measurement surface, for example, coinciding with a second surface that is the lower surface of a measurement stage facing the measuring device. Then, the corresponding at least a first target point, a second target point, and a third target point are positioned on a first surface opposite to the second surface.

[0044] Typically, the method provides a measurement of the position of a first measurement point. For this purpose, the overall structure and shape of the optical element, as well as its refractive index, are known at least to a predefined minimum accuracy. Based on the local profile and shape of the optical element, the position of the first target point on a first surface is determined and / or calculated. For the position of the first measurement point to be measured, it is required that the measurement beam (which undergoes refraction at the first target point) is reflected (e.g., retroreflected) at the first measurement point.

[0045] In order to have effective retroreflection at the first measurement point through the optical medium of the optical element, it is required that the reflected portion of the measurement beam propagating from the first target point toward the first measurement point strikes the second surface substantially perpendicularly, such that the reflected portion of the measurement beam propagates from the first measurement point back to the first target point and overlaps with the measurement beam in space.

[0046] Because the refractive index of the optical element's material differs from that of the surrounding air, the refraction of the measurement beam at the first target point must also be considered. The angle of refraction depends largely on the surface profile in the region of the first target point and the refractive index of the medium of the optical element. In some embodiments, even the radial position of the measuring head is predetermined and / or fixed. For a given radial position of the measuring head relative to the reference axis, it is possible to always set only one pair of defined target points and incident angles. For this purpose, the measurement beam striking the target point is reflected at the target point, enters the medium at the target point, propagates through the medium toward the predetermined first measurement point, and becomes retroreflected at the first measurement point.

[0047] Since the overall shape and profile of the optical element are known with at least minimum precision, and since the refractive index of the medium of the optical element is known, for each of the predefined measurement points of at least the first, second, and third measurement points, it is always possible to calculate and / or determine a specific first target point, second target point, and third target point on the opposite surface of the optical element, respectively.

[0048] Measurement of the positions of at least a first, second, and third measurement points located on a measurement surface opposite the optical head may further include allowing portions of the measurement beam and reflected beam to propagate through a medium (characterized by a known refractive index) of the optical element. Furthermore, the optical paths of the measurement beam and reflected beam portions propagating from the optical head through the medium toward the measurement points and returning to the optical head will be considered for position measurement.

[0049] According to a further embodiment, a first measurement path and a second measurement path are defined on a measurement surface. This can be applied to both a first surface and a second surface, wherein the corresponding first surface and / or second surface is selected as the measurement surface. At least two of a first measurement point, a second measurement point, and a third measurement point are positioned on the first measurement path. At least one of the first measurement point, the second measurement point, and the third measurement point is positioned on the second measurement path.

[0050] By defining a measurement path, multiple corresponding measurement points located along that path can be defined. This improves the accuracy of determining at least one of the eccentricity and tilt of the measurement surface. The more measurement points actually measured, the more accurate the calculation or determination of at least one of the eccentricity and tilt of the corresponding measurement surface will be.

[0051] In some embodiments, the optical element is radially symmetrical. The optical element may include an optical axis. It is particularly advantageous for methods of measuring the surface and / or profile of an optical element when at least two of the first, second, and third measurement points are positioned at radial distances from the optical axis of the optical element that are different from each other.

[0052] Therefore, at least one of the at least three measurement points can be positioned at a first radial distance from the optical axis or radial center point of the optical element. Another of the three measurement points is positioned at a second radial distance from the optical axis or radial center point of the optical element. In this manner, and using at least two measurement points positioned at different radial distances from the optical axis or central axis of the optical element, at least one of the eccentricity and tilt of the measurement surface can be determined based on measurements of the positions of at least the first, second, and third measurement points.

[0053] According to a further embodiment, at least one of the first and second measurement paths is a closed measurement path. The measurement path may include a circular or oval shape, such as an elliptical measurement path. A closed measurement path is particularly advantageous when the optical element is mounted on a rotating measuring stage and the measuring head can be positioned at a variable radial distance from the axis of rotation of the measuring stage.

[0054] For a closed measurement path, measurements at at least the first, second, and third measurement points can begin at any location along the measurement path. Due to the relative motion between the optical elements and the measuring head, the measurement beam can be guided along the measurement path until it returns to its initial position on the path.

[0055] According to another embodiment, the first measurement path and the second measurement path are concentric with the optical axis or reference axis of the optical element (e.g., the reference axis defined by the measuring stage of the measuring device).

[0056] For optical elements such as circular or disk-shaped elements (e.g., lenses), it is very easy to define concentric measurement paths (e.g., concentric measurement rings, wherein at least a first measurement point, a second measurement point, and a third measurement point are located on the concentric measurement ring).

[0057] Typically, the shapes of the first and second measurement paths, as well as their relative positions, can be selected or calculated based on the specific shape or profile of the optical element.

[0058] According to another embodiment, the optical element is attached to a support. The support is arranged on a measuring stage. The measuring stage is rotatable about a rotation axis. At least one of the radial position and orientation of the support relative to the rotation axis is adjustable and is adjusted to minimize at least one of the eccentricity and tilt of the optical element determined by the method of the present invention.

[0059] In some embodiments, the rotation axis of the measuring stage is substantially coincident with or defines the reference axis. In other embodiments, the reference axis is positioned offset from the rotation axis of the measuring stage.

[0060] The support is typically movable radially relative to the axis of rotation. Furthermore, the support is tiltable, or pivotable at least with respect to an axis of tilt relative to the axis of rotation. Typically, the support may be tiltable or pivotable relative to at least two tiltable axes (e.g., in a plane perpendicular to the axis of rotation or perpendicular to a reference axis).

[0061] The support can be adjusted by at least one electric actuator to minimize at least one of the eccentricity and tilt of the optical element relative to a reference axis. In this embodiment, the method provides an automatic adjustment of the support, thereby enabling automatic adjustment of the optical element attached to the support.

[0062] According to another embodiment, in the first measurement procedure, at least one of the eccentricity and tilt of one of the first and second surfaces is determined, and in the second measurement procedure, at least one of the eccentricity and tilt of the other of the first and second surfaces is determined. Between the first and second measurement procedures, the optical element and / or support remains stationary on the measuring stage. The optical element, and thus the support for the optical element, remains stationary relative to the reference axis.

[0063] In this way, the eccentricity and / or tilt determined during the first measurement procedure can be directly associated with and / or mapped to the eccentricity and / or tilt obtained during the second measurement procedure.

[0064] According to another embodiment, a method for measuring the surface and / or profile of an optical element includes measuring the thickness of the optical element along its optical axis. The thickness measurement includes aligning a measuring head along or parallel to the optical axis, guiding the measuring head along or parallel to the optical axis, and moving a focused measuring beam along or parallel to the optical axis, ultimately detecting the coincidence of the convergence area of ​​the focused measuring beam with a first surface and / or a second surface.

[0065] Typically, to measure the thickness of an optical element, a measurement beam is guided along the surface normal of a measurement point positioned on a first surface. A portion of the measurement beam travels through the optical element and is reflected at a corresponding second measurement point on a second surface opposite the first surface. Here, the surface normal of the second point is substantially parallel to the surface normal of the first point. Typically, to measure thickness, the focused measurement beam is moved or scanned longitudinally or axially (thus along the direction of the measurement beam).

[0066] Therefore, the measurement range of the measuring head can be reduced to a range substantially smaller than the expected thickness of the optical element. The intensity of the reflected measuring beam is at its maximum when the focused measuring beam coincides with one of the first and second surfaces. Another maximum intensity of the reflected beam can then be detected by moving the measuring beam (e.g., by scanning the measuring beam along or parallel to the optical axis through the medium of the optical element). Those positions along the optical axis where the local maximum intensity of the reflected beam can be detected indicate that the focused beam coincides with one of the first and second surfaces along the optical axis.

[0067] The position of the measuring head along or parallel to the optical axis when the focused measurement beam coincides with the first surface is compared with the second position of the measuring head along or parallel to the optical axis when the focused measurement beam coincides with the second surface. To calculate the thickness of the optical element, the corresponding first and second positions of the measuring head are compared or subtracted from each other. Here, the refractive index of the medium of the optical element is further considered.

[0068] To distinguish whether the reflected beam is partially reflected from the first surface or the second surface of the optical element, the measurement range of the measuring head should be smaller than the thickness of the optical element. For interferometric measuring devices, the coherence length of the measuring beam should be shorter than or less than the distance along the optical axis c between the first and second surfaces. This can be achieved by utilizing a suitable light source and by using optical delay elements in the optical path of the signal beam and / or the optical path of the reference beam of the interferometric measuring device. For example, a laser or a superluminescent diode can be used as the light source.

[0069] The thickness of an optical element can be measured, even without the first, second, and third positions of the corresponding first, second, and third measurement points on the measurement surface as described above. However, the measurement of thickness is particularly advantageous when the optical element is properly aligned and exhibits minimal eccentricity or tilt relative to the reference axis.

[0070] Therefore, in another aspect, this disclosure relates to a method for measuring the thickness of an optical element along an optical axis (c), the thickness measurement comprising: aligning a measuring head along the optical axis of the optical element; guiding a measuring beam from the measuring head along or parallel to the optical axis; and moving or scanning the focused measuring beam along or parallel to the optical axis.

[0071] During the movement or scanning of the focused measurement beam, the coincidence of the focused area of ​​the measurement beam with the first and / or second surfaces of the optical element is detected. The position and movement of the measuring head along the optical axis are tracked and controlled.

[0072] The distance between the first position of the measuring head when the focused area of ​​the focused measurement beam coincides with the first surface and the second position of the measuring head when the focused area of ​​the focused measurement beam coincides with the second surface directly indicates the thickness of the optical element along the optical axis or in a direction parallel to the optical axis.

[0073] The distance between the first and second positions is further normalized or compensated based on the refractive index of the medium of the optical element.

[0074] The thickness of an optical element can be measured along the optical axis (and thus along the central axis of the optical element). The thickness can also be measured in a direction parallel to the optical axis. Here, the measurement beam is aligned parallel to the optical axis, but positioned radially or laterally off-axis. In this way, the thickness of the optical element can also be measured eccentrically relative to the optical axis. Typically, the thickness of the optical element can be measured at multiple interrelated or corresponding points on the first and second surfaces, respectively. In this way, even the thickness distribution of the optical element can be obtained.

[0075] Typically, the method for measuring thickness can be implemented even without or independently of the measurements of at least the first, second, and third measurement points described above. It can also be performed without first determining the eccentricity or tilt of the measurement surface of the optical element.

[0076] In another embodiment of the method for measuring the surface or profile of an optical element, the optical element is adjusted relative to the interferometric measuring apparatus (especially relative to the measuring head) based on a determined tilt and / or eccentricity. In this way, the tilt and / or eccentricity of the optical element relative to a reference axis, and thus relative to the measuring head, can be eliminated or at least substantially reduced. Only after at least one of the tilt and eccentricity has been eliminated or at least reduced can a high-precision topological measurement of the measurement surface be performed.

[0077] In some embodiments, high-precision topology measurements can be performed by rotating an optical element with a reference axis as the axis of rotation, while simultaneously moving the measuring head of an interferometric measuring device in both the lateral and longitudinal directions, wherein the longitudinal direction extends along or parallel to the axis of rotation, and wherein the lateral direction extends perpendicular to the longitudinal direction. Here, the measuring surface of the optical element can be scanned or probed at multiple measuring points on the measuring surface.

[0078] Furthermore, the measuring head can be oriented toward or aligned with a corresponding measuring point on a measuring surface facing the optical element, thereby causing a portion of the measuring beam to be reflected from the measuring point on the measuring surface toward the measuring head. Typically, the measuring beam is guided substantially parallel to or along the surface normal in the region of the measuring point on the measuring surface.

[0079] Suitable embodiments of high-precision topology measurement are described in more detail in patents US 2017 / 0082521 A1 or DE 10 2011 011 065 B4.

[0080] For the method of measuring the surface of the present invention, high-precision topology measurement of the optical element can preferably be performed only after at least one of the tilt and eccentricity has been determined, and more preferably after the optical element has been aligned relative to the reference axis to eliminate or minimize the tilt and / or eccentricity. Here, the measuring surface undergoing high-precision topology measurement can be one of a first surface and a second surface.

[0081] In some embodiments, the first surface may be the top surface of the optical element facing the measuring head. In some embodiments, the second surface may be the bottom surface of the optical element facing away from the measuring head. Here, high-precision topology measurements can be performed through the optical element. The measuring beam emanating from the measuring head can strike a first target point on the first measuring surface. Subsequently, the measuring beam can propagate through the optical medium and may be reflected at the first measuring point on the second surface of the optical element. Thus, a portion of the measuring beam (typically the portion of the measuring beam reflected at the first measuring point) passes through the optical element back toward the first target point and back toward and / or into the measuring head.

[0082] Here, the optical phase of the returned measurement beam portion can be correlated with and / or compared with the optical phase of a reference beam (e.g., a reference beam generated by the measuring head or a reference beam generated in the measuring head). In this way, the topology of the first and / or second surfaces can be measured with interferometric accuracy without requiring the optical elements to be rearranged, reoriented, or moved relative to the support (configured for holding the optical elements).

[0083] Therefore, in the first step, at least one of the tilt and eccentricity of the first surface can be determined. In the second step, the tilt and eccentricity of the first surface can be substantially eliminated by corresponding adjustments of the optical elements relative to the reference axis and / or relative to the measuring head. In another step, a high-precision topology measurement of the first surface can be performed.

[0084] Subsequently, or in an alternating sequence with the above steps, at least one of the tilt and eccentricity of the second surface of the optical element can be determined by corresponding measurements through the optical element in at least one manner. In this and in another step, the tilt and eccentricity of the first surface can be substantially eliminated by corresponding adjustments of the optical element relative to the corresponding reference axis and / or relative to the measuring head.

[0085] In another step, high-precision topology measurement of the second surface can be performed by propagating a measurement beam through the optical element. Therefore, high-precision measurement of the second surface of the optical element can be provided by scanning through the optical element from the first surface. In effect, high-precision topology measurement of both the first surface and the relatively positioned second surface can be performed without reorienting the optical element relative to the measurement head.

[0086] In another aspect, this disclosure relates to a measuring device for measuring the surface and / or profile of an optical element. The measuring device may be implemented as a non-contact optical measuring device configured to scan the corresponding surface of the optical element. In some embodiments, the measuring device is an interferometric measuring device configured to guide a measurement beam onto the surface of the optical element and detect portions of the beam reflected from the surface of the optical element.

[0087] The measuring apparatus includes a light source configured to generate a measuring beam and guide the measuring beam onto a measuring surface of an optical element. The measuring apparatus further includes a support for fixing the optical element. The measuring apparatus further includes a measuring head connected to the light source. The measuring head is configured to guide the measuring beam onto the measuring surface of the optical element. The measuring head may be further configured to receive a portion of the measuring beam reflected from the measuring surface.

[0088] The measuring head is further movable relative to the support, thereby guiding the measuring beam to at least a predetermined first measuring point, at least a predetermined second measuring point, and at least a predetermined third measuring point on the measuring surface.

[0089] The measuring device further includes a detector connected to the measuring head and configured to detect corresponding portions of the measuring beam reflected at at least at a first measuring point, at least at a second measuring point, and at least at a third measuring point.

[0090] The measuring device further includes a signal analyzer connected to the detector and configured to determine a first position of a first measuring point, a second position of a second measuring point, and a third position of at least a third measuring point. The signal analyzer is further configured to determine at least one of eccentricity and tilt of the measuring surface relative to a reference axis. This determination is based on at least the first, second, and third positions of the corresponding first, second, and third measuring points on the measuring surface.

[0091] Typically, the measuring device is specifically configured to perform the methods described above for measuring the surface and / or contour of optical elements. Within this scope, any features, embodiments, and effects of the methods described above are equally applicable to the measuring device of the present invention; and vice versa.

[0092] To measure or determine at least one of eccentricity and tilt of a measurement surface, a signal analyzer is configured to perform a numerical fitting procedure based on the first, second, and third positions of the corresponding first, second, and third measurement points. The measurement positions are compared with predetermined positions of an optical element, and the profile and geometry of the optical element are known with at least minimum accuracy. Based on the numerical fitting between the measured first, second, and third positions and the predetermined reference geometry of the optical element, at least one of eccentricity and tilt of the corresponding measurement surface relative to a reference axis can be determined.

[0093] Determining at least one of the eccentricity and tilt of the measuring surface includes and / or provides the degree of eccentricity and / or tilt relative to a reference axis. In this way, at least one of the eccentricity and tilt determined by the signal analyzer can be further used to properly align or adjust the optical element relative to the reference axis. This adjustment or calibration can be performed manually or automatically. When performed manually, the entire procedure of measuring the first, second, and third measuring points and obtaining the corresponding first, second, and third positions can be repeated once or multiple times, thereby repeatedly obtaining the corresponding eccentricity and tilt of the measuring surface of the optical element.

[0094] In some embodiments, adjustments to the optical elements can also be performed automatically and / or precisely to align the measuring surface of the optical elements toward a reference axis, for example, by minimizing eccentricity and / or tilt. By precisely measuring the degree or magnitude of eccentricity and / or tilt, the interferometric measuring device (e.g., a controller of the measuring device) can automatically adjust the position and / or orientation of the support of the optical elements, thereby eliminating or at least reducing the eccentricity and / or tilt of the measuring surface relative to the reference axis.

[0095] According to a further embodiment, the support is arranged on a rotatable, and thus rotatable, measuring stage. The rotating measuring stage may define a reference axis. At least one of the radial position of the support and the orientation of the support relative to the reference axis is adjustable. The adjustment of the radial position and / or the orientation of the support can be performed manually. In some embodiments, the rotating measuring stage and / or the support may be provided with at least one or more electromechanical actuators, thereby allowing automatic adjustment of the radial position and / or orientation of the support relative to the reference axis. In this way, very precise and rapid alignment of the support, and thus of the optical element attached to the support, can be provided.

[0096] According to another embodiment, the measuring device includes a controller operable to adjust at least one of the radial position of the support and the orientation of the support relative to a reference axis based on at least one of the eccentricity and tilt of the measuring surface. Here, the controller is particularly configured and adapted to control the operation or actuation of at least one of the actuators of the rotary measuring stage or the corresponding support, thereby moving or orienting the support and / or optical element to a predetermined position and / or orientation relative to the reference axis. In this way, eccentricity and tilt adjustments of the optical element relative to the reference axis based on quasi-automated measurement can be provided.

[0097] Typically, in another embodiment, the operation of the controller can be monitored or controlled by a measuring head while adjusting at least one of the radial position and orientation of the support relative to a reference axis. Therefore, during the adjustment of at least one of the radial position and orientation of the support, the measuring head can be used to control the position of the support and at least one of the optical elements attached to the support. In this way, a feedback loop can be provided during the adjustment or calibration of the support to provide further control over the controller.

[0098] In practice, it can provide very precise and rapid alignment and / or positioning of the support and / or optical elements relative to a reference axis. This is particularly beneficial for reducing the cycle time and clock cycle of subsequent measurements of the surfaces or profiles of multiple optical elements. With the aid of quasi-automatic alignment and adjustment of the support, subsequent high-precision measurements of the profiles and / or surfaces of optical elements can be provided.

[0099] Furthermore, the reduced cycle time and clock cycle for adjustment are beneficial for the use of measuring devices in the quality control of optical components in mass production.

[0100] According to another embodiment, the measuring device further includes a measuring head controller. The measuring head controller is operable to move and / or align the measuring head relative to the support and / or relative to a reference axis. To measure distances from at least a first measuring point, at least a second measuring point, and at least a third measuring point on a measuring surface facing away from the measuring head, the measuring head controller is configured to determine at least a first measuring point, a second measuring point, and a third measuring point on one of the first and second surfaces of the optical element, which is positioned relative to the measuring surface and faces the measuring head.

[0101] Each of the first target point, the second target point, and the third target point is associated with one of the first measurement point, the second measurement point, and the third measurement point, such that the measurement beam entering the medium of the optical element at the first target point, the second target point, and the third target point is retroreflected internally at the first measurement point, the second measurement point, and the third measurement point, respectively.

[0102] Typically, the first target point is directly associated with the first measurement point. The second target point is associated with the second measurement point, and at least the third target point is associated with the third measurement point. After the first target point, the second target point, and at least the third target point have been determined, the measurement head controller is configured to guide or orient the measurement head in a manner such that the measurement beam emitted from the measurement head is directed to the corresponding first target point, the second target point, and at least the third target point, thereby providing measurements of the positions of the associated first measurement point, the second measurement point, and at least the third measurement point.

[0103] In a further embodiment, the measuring device is also configured to perform a high-precision scan of the surface and / or contour of the optical element after the optical element has been aligned relative to a reference axis.

[0104] During scanning or measuring the surface and / or contour of an optical element, the measuring head controller is moved relative to the optical element so that the measuring beam is guided substantially perpendicularly onto the measuring surface of the optical element.

[0105] According to another aspect, this disclosure also relates to a computer program comprising instructions that, when executed by a processor of the measuring apparatus as described above, cause the processor to perform the steps of the method as described above. Within this scope, the methods described above for measuring the surface and / or contour of optical elements, and all the features, effects, and benefits described regarding the measuring apparatus, are equally applicable to the computer program; and vice versa.

[0106] The computer program can be implemented in the processor of the signal analyzer and / or the processor of the measuring head controller of the measuring device. The computer program can be a distributed computer program. One part of the computer program can be executed or deployed in the processor of the signal analyzer. Another part of the computer program can be executed or deployed in the processor of the measuring head controller. Attached Figure Description

[0107] In the following, methods for measuring the surface and / or profile of optical elements, and several embodiments of interferometric measuring apparatus for measuring such surfaces or profiles, are described in more detail with reference to the accompanying drawings, wherein:

[0108] Figure 1 The surface or profile of an optical element is schematically illustrated to determine the eccentricity of the optical element relative to a reference axis;

[0109] Figure 2 It shows that according to Figure 1 A chart showing the measured distances of the layout;

[0110] Figure 3 It shows something similar to Figure 1 In another embodiment, the optical element is eccentric and tilted relative to the reference axis;

[0111] Figure 4 It shows that according to Figure 3 A chart showing the measured distances of the layout;

[0112] Figure 5 An embodiment is shown in which at least a first measurement point, a second measurement point, and a third measurement point are measured on the measurement surface of an optical element, as seen from the side.

[0113] Figure 6 As shown from the top Figure 5 Examples;

[0114] Figure 7 Another embodiment is illustrated, wherein, as seen from the side, the positions of at least three measuring points disposed on the bottom surface of the optical element are measured from above the optical element;

[0115] Figure 8 Shown from the bottom Figure 7 Examples;

[0116] Figure 9 Embodiments of a method and measuring apparatus for measuring the thickness of optical elements are shown;

[0117] Figure 10 It shows that for Figure 9 An example of this embodiment shows the relationship between signal strength and the longitudinal displacement of the measuring head along the optical axis.

[0118] Figure 11 It shows Figure 9 A first configuration of an embodiment, wherein the focusing region of the measurement beam coincides with a first surface;

[0119] Figure 12 It shows Figure 11 In one embodiment, the focusing region of the focused measurement beam coincides with the second surface;

[0120] Figure 13 The configuration of the measuring device during high-precision surface or contour scanning of the first surface is shown;

[0121] Figure 14 Measurements of at least a first measurement point, a second measurement point, and a third measurement point on the second surface of an optical element are shown, wherein the optical element is compared to... Figure 5 The arrangement was flipped;

[0122] Figure 15 It shows in Figure 14 In this configuration, high-precision surface measurement or contour measurement of the second surface is performed.

[0123] Figure 16 It schematically shows the situation according to Figure 14 and Figure 15 In the configuration of the optical elements, measurements are taken through the second surface to at least a first measurement point, a second measurement point, and a third measurement point on the first surface;

[0124] Figure 17 This is a block diagram of the measuring device;

[0125] Figure 18 An embodiment of an adjustable support for optical elements arranged on a rotating measuring stage is shown;

[0126] Figure 19 This is a detailed illustration of one embodiment of an interferometric measuring apparatus; and

[0127] Figure 20 The enlarged diagram shows Figure 19 Some details of the interferometric measuring device;

[0128] Figure 21 This is a flowchart of a method for measuring the surface or profile of an optical element;

[0129] Figure 22 A numerical fitting procedure for obtaining the eccentricity and tilt of the measurement surface is illustrated schematically.

[0130] Figure 23 Another embodiment illustrating the measurement of the thickness of an optical element in the outer edge region is shown schematically. Detailed Implementation

[0131] exist Figure 1 This illustration shows a simplified embodiment of how to measure at least one of eccentricity D and tilt of an optical element 10 relative to a reference axis 51. A rotary measuring stage 50 defining the reference axis 51 is provided. The reference axis 51 may coincide with a rotation axis 53. On top of the measuring stage 50, a support 40 for the optical element 10 (e.g., a lens) is provided. The support 40 is rotatable on the measuring stage 50 about either the reference axis 51 or the rotation axis 53. The optical element 10 is also provided. The optical element 10 is characterized by a measuring surface 11. For the purpose of simple illustration only, the optical element 10 is a cylindrical object and is characterized by cylindrical sidewalls serving as the measuring surface 11.

[0132] The optical element 10 includes an optical axis c. The measuring device 1 includes at least one measuring head 60. The measuring head 60 is configured to measure the distance from the measuring surface 11 when the measuring stage 50 is set to rotate about a reference axis 51 as a rotation axis, thereby moving the support 40 together with the optical element 10. The measuring stage 50 rotates the support 40 and the optical element 10 relative to the reference axis 51 as the rotation axis. Figure 1 As illustrated, the optical axis c of the optical element 10 is located at a radial eccentricity D relative to the reference axis 51.

[0133] exist Figure 1 In the middle, the measuring head 60 can be moved from the upper position to the lower position 60'. Then, in order to measure the tilt T or eccentricity D of the optical element 10, the measuring head 60 measures the distance from the measuring surface 11 along the optical axis c at the first position 60 and measures the distance from the measuring surface 11 along the optical axis at the second position 60'.

[0134] exist Figure 2 In the figure, the distance D from the measuring surface 11 measured by the measuring head 60 is related to the rotation angle of the rotating measuring stage 50. The relationship is illustrated in Figure 70. The distance measured by the measuring head 60' is represented by Figure 70'. Because the geometric center and because the optical axis c is located at a radial deviation D from the reference axis 51 or the rotation axis 53, the distance in the rotation angle according to Figures 70 and 70' is... The amplitude of the optical elements 10 fluctuates. Because the c-axis or central axis of the optical element 10 is parallel to the rotation axis 51, and because the measuring surface 11 is cylindrical and includes a constant diameter along the extension of the optical axis, the slopes and amplitudes of the patterns 70 and 70' are approximately equal. The amplitudes of the patterns 70 and 70' are a direct measurement of the eccentricity D between the optical axis c and the reference axis 51.

[0135] The situation changes when the optical element is skewed or tilted relative to the reference axis 51 or the rotation axis 53. Therefore, as Figure 3 and Figure 4As illustrated, the amplitude of the fluctuation of pattern 70 measured at the position of measuring head 60 is different from the fluctuation or amplitude of pattern 70' measured at another position of measuring head 60'.

[0136] From the changes in amplitude and / or fluctuation of the graph at 70° and 70', at least one of the eccentricity D and tilt angle T of the optical axis c relative to the reference axis 51 can be determined.

[0137] Typically, when the geometry of the optical element 10 is known with at least minimum precision and the measurement points on the measurement surface 11 of the optical element 10 are well defined, the tilt of the measurement surface 11 and the eccentricity of the measurement surface relative to the reference axis 51 can be obtained through numerical analysis. This can be achieved by fitting the actual measurement points of the dedicated measurement points on the measurement surface 11 to a known predetermined reference point of the optical element 10.

[0138] In a more general and common context, the determination of eccentricity and / or tilt is... Figure 5 and Figure 6 Example in.

[0139] exist Figure 5 Side view and Figure 6 In the top view, three dedicated, predefined measurement points 21, 22, and 23 are illustrated. Measurement points 21, 22, and 23 are all radially offset from the central axis or optical axis c of the optical element 10 relative to the optical axis C. The optical element 10 includes a first surface 20 and a relatively positioned second surface 30. In the illustrated embodiment, the optical element 10 may include an optical lens having an upper surface 20 and a lower surface 30. The optical element 10 is made of an optical medium 63 that is transparent to electromagnetic radiation.

[0140] Three dedicated measurement points 21, 22, and 23 are set on the first surface 20, which is currently regarded as the measurement surface 11. These points 21, 22, and 23 are fixed. They can be practically defined on the measurement surface 11. They can be defined in the mathematical model of the optical element 10. They can be identified or defined according to the overall geometry, such as according to the circumferential boundary of one of the first surface 20 and / or the second surface 30, respectively.

[0141] When the first surface 20, and thus the measuring surface 11, is a spherical surface, it is generally sufficient to define at least three measuring points 21, 22, and 23 on the measuring surface 11. Then, the measuring head 60 is used to measure the first position of the first measuring point 21, the second position of the second measuring point 22, and the third position of the third measuring point 23.

[0142] The measurement of the corresponding position is obtained by guiding a measurement beam 61 from the measurement head 60 to, for example, a first measurement point 21. A portion 62 of the measurement beam 61 reflected at the first measurement point 21 is captured and detected by the measurement head 60. The measurement of the first position may include measuring the distance between the measurement head 60 and the first measurement point 21.

[0143] When the measuring device 1 is implemented as an interferometric measuring device, the measuring head 60 can be configured to determine the path difference between the reflected measuring beam portion and the reference beam. The phase shift between the reflected measuring beam portion and the reference beam may indicate the distance between the measuring head 60 and the first measuring point 21. The position and / or orientation of the measuring head 60 in the global coordinate system (e.g., the measuring device) are precisely known.

[0144] Therefore, measuring the distance between the measuring head 60 and the first measuring point 21 allows the position of the first reference point 21 to be determined in the overall coordinate system of the measuring device 1.

[0145] The second position of the second measuring point 22 and the third position of the third measuring point 23 can also be obtained in the same manner. For this purpose, the measuring head 60 undergoes a corresponding movement toward position 60', as follows: Figure 5 As indicated in the document. In addition, the optical element 10 may undergo rotation relative to the reference axis 51.

[0146] After measuring at least the first, second, and third positions of at least the first, second, and third measurement points 21, 22, and 23, the orientation and position of the corresponding measurement surface 11 relative to the reference surface of the optical element can typically be calculated and determined through a numerical fitting operation. The numerical fitting operation is performed by a computer program. Here, the program is provided with structural details and / or geometric data of the optical element 10. The geometric data of the optical element 10 can be stored in the computer program as a reference optical element perfectly aligned with the reference axis 51. Now, by fitting the at least first, second, and third positions actually measured by the measuring heads 60, 60' to the numerical model of the reference optical element or reference measurement surface, the eccentricity and / or tilt angle of the measurement surface 11 relative to the corresponding reference surface or reference axis can be numerically determined.

[0147] exist Figure 22In this diagram, the determination of the eccentricity D and tilt T of the measuring surface 11 is visualized based on numerical fitting with the reference surface 11'. There, the axial position of the optical element 10 and the first, second, and third measuring points 21, 22, and 23 on the measuring surface 11 are schematically illustrated. Because the overall shape and geometry of the optical element 10 are known with minimum accuracy based on position measurements at at least three measuring points 21, 22, and 23, the orientation and position of the measuring surface 11 can be characterized and determined through a numerical fitting procedure.

[0148] The reference surface 11' of the reference optical element 10' is further illustrated by dashed lines. The reference optical element 10' represents the position and orientation of the optical element 10 when it is fully aligned with the subsequent high-precision surface measurement procedure. The first, second, and third positions of at least the first, second, and third measurement points 21, 22, and 23 are numerically fitted to a mathematical model with eccentricity D and tilt angle T as variables. The measured positions of at least the first, second, and third measurement points 21, 22, and 23 are numerically fitted to minimize deviations from the reference surface 11'. This results in the numerical determination of the corresponding tilt angle T and the corresponding radial eccentricity D.

[0149] In this manner, and by probing at least the first, second, and third positions of the dedicated, predefined first, second, and third measurement points 21, 22, 23 on the measurement surface 11 of the optical element 10, the eccentricity D and / or tilt of the measurement surface 11 relative to the reference axis 51 can be determined. For a spherical measurement surface 11, three discrete measurement points 21, 22, 23 may be sufficient. If the measurement surface 11 comprises an aspherical shape, at least five dedicated, predefined measurement points are required on the measurement surface. In the case that the measurement surface is a freeform surface, at least six dedicated and / or predefined measurement points 21, 22, 23 are required.

[0150] In this embodiment, a first measurement path 25 and a second measurement path 29 are provided on the measurement surface 11. In the currently illustrated embodiment, measurement paths 25 and 29 are both closed measurement paths. On the measurement surface 11, the first measurement path 25 may include a circle or an ellipse. Furthermore, the second measurement path 29 may also include a circle or an ellipse. As further illustrated, the first measurement point and the second measurement points 21 and 22 are located on the first measurement path 25. Only the third measurement point 23 is located on the second measurement path 29.

[0151] By selecting or defining at least a first measurement path and a second measurement path 25, 29 on the measurement surface 11, and by utilizing a large number of measurement points on at least two measurement paths 25, 29, the positions of a relatively large number of measurement points can be determined. The measuring head 60 can scan along the first measurement path 25 and / or along the second measurement path 29, and may thus determine the positions of a corresponding number of measurement points on the measurement surface 11. Generally, the more measurement point positions obtained, the more accurate the fitting procedure for determining at least one of the eccentricity and tilt of the measurement surface relative to the reference axis 51 may be.

[0152] exist Figure 7 and Figure 8 In this embodiment, the second surface 30 of the optical element 10 is defined as a measurement surface 11, as illustrated in the bottom view of the optical element 10, and the measurement surface 11 is further provided with a first measurement point 31, a second measurement point 32, and a third measurement point 33. Here, the first and second measurement points 31 and 32 are located on a first measurement path 35. The third measurement point 33 is located on a second measurement path 39.

[0153] In the currently illustrated embodiment, the first and second measurement paths 25, 35, 29, 39 are concentric. However, they can also be disconcentric or skewed relative to each other. Furthermore, it is conceivable that measurement paths 25, 29, 35, 39 intersect each other.

[0154] To measure the positions of the first, second, and third measurement points 31, 32, and 33, the measuring head 60 remains positioned on one side of the first surface 20 of the optical element. Figure 7 As indicated, the positions of the first measurement point, the second measurement point, and the third measurement point 31, 32, and 33 are measured through the medium 63 of the optical element 10. To accurately measure the first position of the first measurement point 31, the second position of the second measurement point 32, and the third position of the third measurement point 33, corresponding first target points 26, 27, and 28 are defined on opposite sides of the optical element 10, thereby on the first surface 20. The first target point 26 is directly related to the first measurement point 31. The second target point 27 is directly related to the second measurement point 32, and the third target point 28 is directly related to the third measurement point 33.

[0155] Because the measurement of the positions of multiple measurement points 31, 32, 33, and thus the measurement of the distance between the measurement heads 60, 60' and the measurement points 31, 32, 33, includes the propagation of the measurement beam 61 through the medium 63, the corresponding target points 26, 27, 28 are calculated and / or determined such that the measurement beam 61 guided to the first target point 26 is refracted at the first target point, so that the refracted portion 61' of the measurement beam is retroreflected at the first measurement point 31 on the second surface 30. Then, the retroreflected portion 62' of the measurement beam propagating through the medium 63 undergoes refraction at the first surface 20, and as the reflected portion 62, it re-enters the measurement head 60 in the direction opposite to the measurement beam 61.

[0156] Target points 26, 27, and 28 are selected and determined based on the refractive index of medium 63 and the incident angle of measurement beam 61 at the corresponding target points 26, 27, and 28. In addition, the selection and determination of target points also take into account the slope or surface profile of the regions of measurement points 31, 32, and 33 and / or target points 26, 27, and 28, as well as the thickness or profile of optical element 10.

[0157] In some embodiments, the determination of the calculation of target points 26, 27, 28 on the first surface (e.g., the surface facing the measuring head 60) may also take into account at least one of the positions or orientations of the measuring heads 60, 60'.

[0158] In addition, in order to determine the optical path length between the measuring heads 60, 60' and the measuring points 31, 32, 33 on the surface 30 of the optical element 10 opposite to the measuring heads 60, 60', the geometry of the optical path, the refractive index, and the path length of the beam propagating through the medium 63 are taken into account.

[0159] Since the refractive index of medium 63 and the geometry and profile of optical element 10 are known with minimum precision, the eccentricity D and / or tilt of the corresponding measuring surface 11 relative to the reference axis 51 can be determined with at least sufficient precision to determine the positions of measuring points 31, 32, and 33.

[0160] Measuring multiple measurement points 21, 22, 23 on the measurement surface 11 coinciding with the first surface of the optical element 10 allows for the determination of at least one of the eccentricity and tilt of the first surface 20 relative to a reference axis. This determination or measurement constitutes a first measurement procedure. Measurements of the first, second, and third measurement points 31, 32, 33 on the second surface 30 of the optical element 10 constitute a second measurement procedure. When from, as Figure 5 The first measurement procedure illustrated in the example switches to, as shown in the example. Figure 7 During the second measurement procedure illustrated herein, the position of the optical element 10 on the support 40 and / or the measuring stage 50 may remain unchanged.

[0161] Within this scope, two measurement procedures are performed sequentially, with one procedure using the first surface 20 of the optical element as the measurement surface 11 and the other using the second surface 30 of the optical element 10 as the measurement surface 11 directly and intrinsically correlated with each other. The eccentricity and tilt of the first surface 20 obtained by the first measurement procedure can be directly correlated and mapped to the eccentricity and tilt of the second surface 30 obtained by the second measurement procedure. It is no longer necessary to flip or twist the support 40 and / or the optical element 10 to determine the eccentricity and tilt of the relative positioning surfaces 20, 30 of the optical element 10.

[0162] Therefore, direct mapping and assignment of the geometric data and characteristics of the relative positioning surfaces 20, 30 of the optical element 10 can be obtained without reorienting or flipping the optical element 10 or the support 40 between successive measurement procedures.

[0163] exist Figures 9 to 12 The sequence illustrates the measurement of the thickness of the optical element 10. For this purpose, the measuring head 60, emitting the measuring beam 61, is aligned along the optical axis c of the optical element 10. This alignment can be performed after the eccentricity D and tilt T of the optical element 10 relative to the reference axis 51 have been determined. The measuring beam 60 is then focused along the optical axis c.

[0164] Now, the measuring head 60 and the focused measuring beam 61 can be moved along the optical axis. When the focused area 68 of the focused measuring beam 61 coincides with the first surface 20 of the optical element 10 (e.g., Figure 11 As illustrated in the example, the intensity or signal strength of the reflected beam portion detected and / or captured by the measuring head 60 will be at a maximum value M1 (as shown in the example). Figure 10 (As indicated in the document). This maximum value M1 is obtained when the measuring head 60 is in the first measuring position z1.

[0165] When the measuring head 60' moves along the optical axis (as from...) Figure 11 The location indicated in the text faces the direction as follows Figure 12 When the position shown is such that the focal region 68 of the measurement beam 61' is close to the second surface 30 of the optical element, the second maximum value M2 of the captured and reflected measurement beam is generated. This maximum value M2 is obtained when the measuring head 60' is located at the second axial position z2.

[0166] The difference or distance between positions z1 and z2 directly indicates the thickness of optical element 10 along the optical axis c. The refractive index of medium 63 is also considered to determine the thickness accurately.

[0167] exist Figure 23 Another embodiment illustrating the measurement of the thickness of an optical element 10 at a location offset from or c-axis but parallel to the optical axis is shown. Here, a measurement beam 61 is guided onto an edge portion 72 of the optical element 10. The edge portion 72 is the radially outer edge portion of the optical element (e.g., an optical lens). A first surface 20 and a second surface 30 in the edge portion 72 extend substantially parallel to each other. The measurement beam 61 is guided parallel to the optical axis c to a first measurement point 74 in the edge portion 72 on the first surface 20. At least a portion 61' of the measurement beam 61 is transmitted through a medium 63 and is retroreflected at a second measurement point 75, which is relatively positioned in the edge portion 72 on the second surface 30. The reflected measurement beam portions 62' and 62 return from there and enter the measurement head 60.

[0168] As mentioned above Figures 9 to 12 As described, the position of the measuring head 60 varies or moves along the direction of the measurement beam propagation, typically along or parallel to the optical axis c, thereby detecting the local maximum intensity of the reflected light at the first measuring point 74 and / or the second measuring point 75, respectively.

[0169] In this way, the method for determining or measuring the thickness of an optical element is not limited to measurements along the optical axis c. The measurements described above can be applied to any region of the optical element, where the surface normal of a first point on a first surface extends substantially parallel to the surface normal of a second point on a relatively positioned second surface. Typically, for optical lenses, this requirement is generally met in the c-axis region.

[0170] After determining the eccentricity D and / or tilt T of the measuring surface 11 relative to the reference axis 51 (e.g., the rotation axis 53 relative to the measuring stage 50), the measuring surface 11 (e.g., the first surface 20) can be precisely measured by scanning the measuring head 60 on the measuring surface 11 using multiple positions 60, 60', and 60" as indicated. Figure 13 As shown. This surface measurement conforms very well to the measurement procedures described in documents US2017 / 0082521 A1 or DE 10 2011 011 065 A1, which are incorporated herein by reference in their entirety.

[0171] Figure 14 The embodiments illustrated herein are in some respects equivalent to Figure 5 The embodiment illustrated herein, except that the optical element 10 is flipped so that the second surface 30 faces upward toward the measuring head 60. Here, the measuring surface 11 coinciding with the second surface 30, and the first measuring point, second measuring point, and third measuring point 31, 32, 33 on the second surface 30, are referenced above. Figure 5 The same methods described are used for measurement and detection.

[0172] After determining at least one of the tilt T and eccentricity D of the measuring surface 11 of the second surface 30, high-precision surface scanning or contour scanning of the second optical surface 30 can also be performed as described above. Figure 13 The execution described.

[0173] It is also conceivable that target points 36, 37, and 38 are defined on the second surface 30, such as... Figure 16 As illustrated in the example, measurement points 21, 22, and 23 set on the first surface 20 (now facing away from the measuring head 60) can be measured in this manner. Here, the positions of the first, second, and third measurement points 21, 22, and 23 on the first surface 20 can be measured by defining corresponding first, second, and third target points 36, 37, and 38 on the upward-facing second surface 30, and by passing through the medium 63 of the optical element. This method increases measurement accuracy.

[0174] Through such Figure 5 The measurement procedure illustrated herein can directly determine at least one of the eccentricity and tilt of the first surface 20. Figure 16 In this configuration, the eccentricity and / or tilt of the first surface 20 can be measured via the medium 63 and by defining target points 36, 37, and 38 on the relatively positioned second surface 30. (As per...) Figure 5 The determination of the eccentricity and / or tilt of the first surface 20 obtained by the measurement procedure can be compared with, for example, Figure 16 The measurements of eccentricity and / or tilt obtained from the measurement procedure in the configuration of the optical elements illustrated are compared and correlated. In this way, the eccentricity and tilt of both the first surface 20 and the second surface 30 can be measured twice in two different ways, thereby improving the accuracy of determining the eccentricity and / or tilt of the corresponding surfaces 20 and 30 of the optical element 10.

[0175] exist Figure 17 The diagram illustrates one embodiment of the measuring device 1. The measuring device 1 includes a light source 2, an optical coupler 3, a detector 4, and a measuring head 60. The measuring head 60 is optically coupled to the light source 2 via an optical fiber 7. A measuring beam 61 generated by the light source 2 can be guided to the measuring head 60 via the optical coupler 3 and the optical fiber 7. When the measuring device 1 is implemented as an interferometric measuring device, the measuring beam 61 is split into a signal beam and a reference beam guided to the optical element 10. Some embodiments are described in more detail, for example, in at least one of the documents DE 10 2011 011 065 B4 or US 2017 / 0082521 A1.

[0176] A reference beam can be generated at the fiber exit face located inside the measuring head 60. A portion 62 of the measuring beam reflected from surfaces 20 and 30 of the optical element 10 is captured by the measuring head 60 and propagates along with the reference beam toward the optical coupler 3 in the optical fiber 7. In a typical embodiment, the optical coupler 3 includes an optical circulator.

[0177] Light propagating from the measuring head 60 toward the optical coupler 3 is redirected to the detector 4. The detector 4 includes multiple photosensitive elements (such as an array or matrix of charge-coupled devices) to detect an interference pattern generated by the interference of a reference beam with a portion 62 of the measuring beam that is reflected at one of the surfaces 20, 30 of the optical element 10. The detector 4 is connected to a signal analyzer 5 to resolve and / or determine the relative phase between the reflected signal beam and the reference beam.

[0178] Typically, the signal analyzer 5 includes a processor 8 to calculate or determine the relative phase, thereby obtaining and / or capturing the optical path difference between the reflected signal beam and the reference beam via the measuring head 60. Based on the optical path difference, the distance to selected points on the surfaces 20, 30 of the optical element 10 can be determined. By further understanding the precise position of the measuring head relative to the overall coordinate system of the measuring device 1, the positions of the corresponding measuring points on the surfaces 20, 30 of the optical element 10 can be obtained.

[0179] As described above, an optical element 10 (e.g., in the form of a lens) is mounted on a support 40. The support 40 is rotatably supported on a rotary measuring stage 50. The measuring stage 50 defines a reference axis 51. The reference axis 51 may coincide with a rotation axis 53 defined by the measuring stage 50.

[0180] The measuring device 1 further includes a measuring head controller 66. The measuring head controller 66 includes at least one processor 9. The measuring head controller typically controls and manages the position and orientation of the measuring head 60. The signal analyzer 5 and detector 4 may be implemented as integrated components of the controller 6 of the measuring device 1. In this way, the measuring head controller 66 may also be implemented as a single component, for example, as an integral part of the controller 6. The processors 8 and 9 illustrated herein may also be integrated into a single processing unit of the controller 6.

[0181] The measuring head controller 66 can also be implemented as a discrete controller. The controller 6 is configured to control or communicate with the measuring head controller 66. In this way, the controller 6 is configured to determine the measurement points to be scanned on the surfaces 20, 30 of the optical element 10, and to dispatch the portion 62 of the measuring beam captured from the corresponding measurement point to the corresponding measurement point on the surfaces 20, 30.

[0182] In some embodiments, the support 40 is controllable by a controller 6. Therefore, the controller 6 can be configured to orient or move the support 40, and thus the optical element 10, relative to the reference axis 51. In this way, the controller 6 can be configured to automatically adjust the eccentricity and / or tilt of the optical element 10 relative to the reference axis 51. This embodiment is described based on cylindrical coordinates. Because the support 40 is rotatable relative to the rotation axis 53, and thus relative to the reference axis 51, the support 40 is radially movable relative to the fixed measuring stage 50. The support 40 can also be tilted relative to at least a first tilt axis a and a second tilt axis b. The tilt axes a and b can extend in a plane perpendicular to the reference axis 51 or perpendicular to the rotation axis 53. The tilt axes a and b can be fixed relative to the support 40. In some embodiments, the tilt axes a and b may be reconfigurable. Therefore, the position and / or orientation may change.

[0183] exist Figures 18 to 20 In one embodiment, the support 40 includes a base 41 that can be fixed to the measuring stage 50. A middle portion 42 is positionable and rests on the upper surface of the base 41. An upper portion 44 is disposed on the upper surface 43 of the middle portion 42. The middle portion 42 is loosely mounted on the upper surface of the base 41. It can engage frictionally with the upper surface of the base 41. The lower or bottom surface of the middle portion 42 can be flat and can make surface contact with the complementary flat upper surface of the base 41.

[0184] In this manner, and because the reference axis 51 or the rotation axis 53 extends substantially perpendicularly through the flat-shaped surfaces of the intermediate portion 42 and the base 41, the intermediate portion 42 is movable radially r relative to the base 41. The intermediate portion 41 is radially movable by an actuator. The actuator 47 may include a mechanical pulse generator or pulsating device configured to apply a force radially to the intermediate portion 42. In this way, the intermediate portion 42 can undergo radial displacement relative to the base 41.

[0185] The upper surface 43 of the middle portion 42 is dome-shaped. The lower surface 45 of the upper portion 44 is a complementary dome shape. Therefore, the upper surface 43 may include a concave shape, and the lower surface 45 of the upper portion 44 may include a convex shape of a corresponding or complementary shape. The functions of the convex upper surface and the concave lower surface of the middle portion 42 and the upper portion 43 are also interchangeable. Typically, the dome-shaped surfaces 43 and 45 include both convex and concave shapes in both transverse directions relative to the rotation axis 51.

[0186] A further actuator 48 is provided, configured to selectively engage with the upper portion 44. Furthermore, the actuator 48 may include a pulse generator or pulsation device configured to repeatedly apply momentum to the outer edge or side surface of the upper portion 44. In this manner, and due to the corresponding dome-shaped surfaces 45, 43, the upper portion 44 can be tilted relative to the middle portion 42 as the dome-shaped surface 45 slides within the corresponding dome-shaped surface 43. The dome-shaped surfaces 43, 45 are frictionally engaged and maintain their mutual orientation, even under the influence of gravity and when undergoing rotation relative to the rotation axis 51.

[0187] Once the eccentricity D and / or tilt T of the measuring surface 11 have been determined, the controller 6 can be configured to adjust the alignment or positioning of the support 40 relative to the reference axis 51. Therefore, the controller 6 can control and activate actuators 47, 48 to align and position the optical element 10, for example... Figure 13 or Figure 15 The subsequent high-precision surface scanning procedure is shown. During the operation of actuators 47, 48, the position and / or orientation of optical element 10 can be monitored by measuring head 60. In this way, the distance or position measurement provided by measuring head 60 forms a feedback loop during the movement or alignment of support 40. This can be particularly beneficial for the high-precision automatic adjustment of support calibration for high-precision surface measurement procedures.

[0188] exist Figure 20 The actual implementation of the support 40 is illustrated herein. As illustrated, the upper portion 44 is provided with a chamfer 49 along its outer edge and / or upper edge. An actuator 48 is configured to apply momentum to the upper portion 44 and is aligned relative to the chamfer 49, thereby striking or impacting the chamfer 49 substantially perpendicularly. Another actuator 47 is horizontally aligned and is thus configured to apply radial momentum to the intermediate portion 42.

[0189] exist Figure 19An embodiment of measuring device 1 is illustrated below. Measuring device 1 is closely related to devices explained and described in more detail, for example, in document DE 10 2011011 065 B4 or US 2017 / 0082521 A1. Measuring device 1 includes a fixed base 81. On the opposite lateral side of the base 81, there are upwardly extending legs 83. The upper ends of the legs 83 are connected by a through portion 84. The crossbeam 84 and the legs 83 form a frame 82 attached to the base 81. A rotating measuring stage 50 is located on the bottom of the base 81. The base 81 may additionally include an upwardly pointing or upwardly extending back side 85. A support 90 and a distance measuring device 92 are disposed on the back side 85. The support 90 is movable with respect to at least two longitudinal directions, for example along the horizontal direction (x) and the vertical direction (z). The support 90 is also movable along a second horizontal direction, the y-direction, relative to the rotating stage 50. The x and y directions can form a radial plane perpendicular to the rotation axis 53 or the reference axis 51.

[0190] The bracket 90 is further provided with a support portion 91. The support portion 91 is rotatably mounted on the bracket 90. Typically, the support portion 91 may include or define a rotation axis extending along the y-direction. A reference body 95 is further provided on the bracket 90. The reference body 95 includes a reference surface 96 facing the distance measuring device 92. The distance measuring device 92 includes at least one distance sensor 93. The distance sensor 93 faces the support 40 and thus faces the optical element 10 located on the support 40. The optical element 10 may be positioned on a support 46. The support 46 may be positioned on the upper portion 44 of the support 40. In some embodiments, the support 46 includes a hydraulically expanded chuck to allow for securing or firmly clamping the optical element 10.

[0191] The position of the distance measuring device 92 in the overall coordinate system of the measuring device 1 can be precisely determined by at least a first reference sensor 88 pointing toward the first reference surface 86. The reference surface 86 extends vertically, thus along the z-direction, and is attached to one of the upward-pointing legs 83. A second reference sensor 89 may face another reference surface 87 disposed on the crossbeam 84. The reference surface 87 extends horizontally, for example, along the x-direction.

[0192] Reference sensors 88 and 89 are positioned and fixed on bracket 90. Reference sensors 88 and 89 are configured to determine the position of bracket 90 in the xz plane. Both sensors 88 and 89 can be implemented as distance sensors. Reference sensors 88 and 89 are configured to determine the distances to the corresponding calibration reference surfaces 86 and 87, respectively.

[0193] The distance measuring device 92 is rotatably mounted on the bracket 90 and pivotable relative to a rotation axis extending substantially along the y-direction. The distance measuring device 92 includes a first distance sensor 93 facing the bracket 40 and thus facing the optical element 10. The distance measuring device 92 further includes a second distance sensor 94 facing a reference surface 96 of a reference body 95 fixed to the bracket 90.

[0194] In the currently illustrated embodiment, the first reference sensor 93 and the second reference sensor 94 extend in opposing directions, such as radially opposing directions. The second distance sensor 94 is configured to determine the distance between the distance measuring device 92 and the reference surface 96. In this way, any positional changes of the distance measuring device 92 that may be caused by rotation of the distance measuring device 92 relative to the support 90 can be accurately compensated for and tracked.

[0195] The operation of the measuring device 1 and the method for measuring the surfaces 20, 30 or contours of the optical element 10 involve multiple steps in Figure 21 The flowchart is further described. In the first step 100, a plurality of measurement points 21, 22, 23 are defined on the measurement surface 11 of the optical element 10, for example, at least three. The total number of predefined measurement points can vary depending on the type of the measurement surface 11. For a typical embodiment, a very large number of measurement points 21, 22, 23 are defined located on at least two measurement paths 25, 29, such as Figure 5 and Figure 6 exemplified in .

[0196] Subsequently, in step 102, the measuring head 60 is moved relative to the optical element 10, thereby scanning along the measuring paths 25, 29. At least the measuring device 1 operates in this manner to obtain first, second, and third positions of a plurality of measuring points 21, 22, 23. Based on the position measurements obtained in step 102, in the subsequent step 104, at least one of the eccentricity D and tilt T of the measuring surface 11 relative to the reference axis 51 is determined.

[0197] Based on the determined tilt T and / or eccentricity D, the optical element 10 is adjusted in step 106.

[0198] Precise adjustments, thereby eliminating or significantly reducing tilt T and / or eccentricity D, are particularly beneficial for subsequent high-precision topological measurements of the measurement surface 11 performed in step 108. Typically, this is achieved by rotating the optical element 10 with a rotary measuring stage 50 and by scanning at least a portion or the entirety of at least one of the measurement surfaces 20, 30, such as... Figure 13Or, as illustrated in 15, topology or surface measurement is performed by optical element 10. During or for high-precision topology measurement of the surface, the measuring head 60 and the corresponding measuring beam 61 are focused particularly on surfaces 20, 30 facing the measuring head 60.

[0199] Once topology measurements have been performed (such as, for example) Figure 13 (As illustrated in the example) After that, the measuring head 60 can be as follows Figure 9 The alignment is shown along the optical axis c of the optical element 10. Subsequently, in step 108, the measuring head 60 can be moved along the optical axis c to perform a thickness measurement, as illustrated. Figures 9 to 12 exemplified.

[0200] To perform thickness measurements, the measuring range of the measuring head should be less than the thickness of the optical element 10. For the interferometric measuring apparatus, the coherence length of the measuring beam should be shorter than or less than the distance along the optical axis c between the first surface 20 and the second surface 30. This can be achieved by using a suitable light source 2 and by using optical delay elements in the optical path of the signal beam and / or the optical path of the reference beam of the interferometric measuring apparatus 1.

[0201] Label Explanation

[0202] 1. Measuring device 45. Dome-shaped surface

[0203] 2 Light source 46 Support

[0204] 3 Optical Coupler 47 Actuator

[0205] 4 detectors 48 actuators

[0206] 5. Signal analyzer with 49 chamfers

[0207] 6 controllers, 50 measuring stations

[0208] 7 Fiber Optic 51 Reference Axis

[0209] 8 processors, 53 rotating axes

[0210] 9 processors, 60 measuring heads

[0211] 10 Optical components 61 Measuring beam

[0212] 11 Measuring surface 62 Measuring beam section

[0213] 20 Surface 63 Medium

[0214] 21 Measurement points 66 Measurement head controller

[0215] 22 measurement points, 68 focal areas

[0216] 23 Measurement Points 70 Graphics

[0217] 25 Measurement path 72 Edge section

[0218] 26 Target Points 74 Measurement Points

[0219] 27 Target Points 75 Measurement Points

[0220] 28 Target Point 81 Base

[0221] 29 Measurement Path 82 Frame

[0222] 30 Surface 83 Legs

[0223] 31 Measurement points 84 Crossbeam

[0224] 32 Measurement points 85 Backside

[0225] 33 Measurement points 86 Reference surface

[0226] 35 Measurement Path 87 Reference Sensor

[0227] 36 target points, 88 reference surfaces

[0228] 37 Target Points 89 Reference Sensors

[0229] 38 Target Point 90 Support

[0230] 39 Measurement Path 91 Support Section

[0231] 40 Supports 92 Distance Measuring Device

[0232] 41 Base 93 Distance Sensor

[0233] 42 Middle section 94 Distance sensor

[0234] 43 Dome-shaped surface 95 Reference body

[0235] 44 Upper 96 Reference Surface

Claims

1. A method for measuring the surface (20, 30) or profile (P) of an optical element (10), wherein the optical element (10) is fixed to a support (40) arranged on a rotary measuring stage (50) defining a reference axis (51), wherein the optical element (10) includes a first surface (20) and a second surface (30) opposite to the first surface (20), the method comprising the steps of: - At least a first measurement point (21, 31), a second measurement point (22, 32), and a third measurement point (23, 33) are defined on the measurement surface (11) of the optical element (10), wherein the measurement surface (20, 30) is one of the first surface (20) and the second surface (30). - The first position of the first measurement point (21, 31) is measured by guiding the measurement beam (61) from the measurement head (60) to the first measurement point (21, 31) and by detecting the portion (62) of the measurement beam reflected at the first measurement point (21, 31). - Subsequently, by guiding the measuring beam (61) to the second measuring point (22, 32) and the third measuring point (23, 33) respectively, and by detecting the portion (62) of the measuring beam reflected at the second measuring point (22, 32) and the third measuring point (23, 33) respectively, at least the second position of the second measuring point (22, 32) and the third measuring point (23, 33) are measured. - Based on at least the first position, the second position, and the third position - i) Determine at least one of the eccentricity (D) and tilt (T) of the measuring surface (11) relative to the reference axis (51), and / or - ii) Derive the measured surface or the measured profile of the optical element (10); One of the first surface (20) and the second surface (30) of the optical element (10) faces the measuring head (60), and the other of the first surface (20) and the second surface (30) facing away from the measuring head (60) is the measuring surface (11), and the first position, the second position and the third position are measured by guiding the measuring beam (61) to the first target point (26, 36), the second target point (27, 37) and the third target point (28, 38), and the first target point (26, 36), the second target point (27, 37) and the third target point (28, 38) are located on one of the first surface (20) and the second surface (30) facing the measuring head (60), and the measuring beam (61) propagates through the medium (63) of the optical element (10).

2. The method according to claim 1, wherein measuring at least one of the first position, the second position and the third position comprises focusing the measuring beam (61) onto at least one of the first measuring point (21, 31), the second measuring point (22, 32) and the third measuring point (23, 33), and detecting the corresponding portion (62) of the measuring beam reflected backward at at least one of the first measuring point (21, 31), the second measuring point (22, 32) and the third measuring point (23, 33).

3. The method according to claim 1, wherein the positions of the first target point (26, 36), the second target point (27, 37), and the third target point (28, 38) located on one of the first surface (20) and the second surface (30) facing the measuring head (60) are determined based on the following: - The refractive index of the medium (63) of the optical element (10), - The angles of incidence of the measuring beam (61) at the first target point (26, 36), the second target point (27, 37), and the third target point (28, 38), and - A partial surface profile of at least one of the first surface (20) and the second surface (30) in the region of the first measurement point (21, 31), the second measurement point (22, 32) or the third measurement point (23, 33) and / or the first target point (26, 36), the second target point (27, 37) and the third target point (28, 38).

4. The method according to any one of claims 1 to 3, wherein a first measurement path (25, 35) and a second measurement path (29, 39) are defined on the measurement surface (11), wherein at least two of the first measurement point (21, 31), the second measurement point (22, 32) or the third measurement point (23, 33) are located on the first measurement path (25, 35), and at least one of the first measurement point (21, 31), the second measurement point (22, 32) or the third measurement point (23, 33) is located on the second measurement path (29, 39).

5. The method according to claim 4, wherein at least one of the first measurement path (25, 35) and the second measurement path (29, 39) is a closed measurement path.

6. The method according to claim 4, wherein the first measurement path (25, 35) and the second measurement path (29, 39) are concentric with respect to the optical axis (c) of the optical element (10) or with respect to the reference axis (51).

7. The method according to any one of claims 1 to 3, wherein the measuring stage (50) is rotatable relative to the rotation axis (53), wherein at least one of the radial position (r) and orientation (a, b) of the support (40) relative to the rotation axis (53) is adjusted to minimize at least one of the eccentricity (D) and tilt (T) of the optical element (10).

8. The method according to any one of claims 1 to 3, wherein in a first measurement procedure, at least one of the eccentricity (D) and the tilt (T) of one of the first surface (20) and the second surface (30) is determined, and in a second measurement procedure, at least one of the eccentricity (D) and the tilt (T) of the other surface of the first surface (20) and the second surface (30) is determined.

9. A method for measuring the thickness of an optical element (10) along an optical axis (c), the thickness measurement comprising: - The measuring head (60) is aligned with the optical axis (c) of the optical element (10) to emit the measuring beam (61); - Focus the measurement beam (61) along or parallel to the optical axis (c) and guide the focused measurement beam (61). - Moving the focused measurement beam (61) along or parallel to the optical axis (c); and - Detect the overlap between the focal region (68) of the focused measurement beam (61) and the first surface (20) and / or the second surface (30) of the optical element (10), wherein the second surface (30) is opposite to the first surface (20); The method further includes the step of detecting the maximum value of the intensity of the reflected beam when the focused measurement beam (61) coincides with the first surface (20) and the second surface (30), respectively; and wherein the thickness of the optical element (10) is calculated by comparing or subtracting a first position of the measuring head (60) along or parallel to the optical axis (c) when the focused measurement beam (61) coincides with the first surface (20) and a second position of the measuring head (60) along or parallel to the optical axis (c) when the focused measurement beam (61) coincides with the second surface (30).

10. The method according to claim 9, wherein the focused measurement beam (61) is scanned in a longitudinal direction along the direction of the measurement beam (61).

11. An interferometric measuring apparatus (1) for measuring the surface (20, 30) or profile of an optical element (10), the measuring apparatus comprising: - Light source (2), which is configured to emit a measurement beam (61); - A support (40) for fixing the optical element (10), wherein the support (40) is arranged on a rotary measuring stage (50) that defines a reference axis (51). - A measuring head (60) is connected to the light source (2), the measuring head (60) is configured to guide the measuring beam (61) onto the measuring surface (11) of the optical element (10), and is further configured to receive a portion (62) of the measuring beam reflected from the measuring surface (11). - The measuring head (60) is movable relative to the support (40) to guide the measuring beam (61) to at least a predetermined first measuring point (21, 31), a predetermined second measuring point (22, 32), and a predetermined third measuring point (23, 33) on the measuring surface (11); - Detector (4), which is connected to the measuring head (60) and configured to detect the portion (62) of the measuring beam reflected at least at the first measuring point (21, 31), the second measuring point (22, 32) and the third measuring point (23, 33). - A signal analyzer (5), which is connected to the detector (4) and configured to determine a first position of the first measurement point (21, 31), a second position of the second measurement point (22, 32) and a third position of at least the third measurement point (23, 33), the signal analyzer (5) being further configured to i) determine at least one of the eccentricity (D) and tilt (T) of the measurement surface (11) relative to the reference axis (51) based on at least the first position, the second position and the third position, and / or ii) derive the measured surface or the measured profile of the optical element (10); One of the first surface (20) and the second surface (30) of the optical element (10) faces the measuring head (60), and the other of the first surface (20) and the second surface (30) facing away from the measuring head (60) is the measuring surface (11), and the first position, the second position and the third position are measured by guiding the measuring beam (61) to the first target point (26, 36), the second target point (27, 37) and the third target point (28, 38), and the first target point (26, 36), the second target point (27, 37) and the third target point (28, 38) are located on one of the first surface (20) and the second surface (30) facing the measuring head (60), and the measuring beam (61) propagates through the medium (63) of the optical element (10).

12. The measuring device (1) according to claim 11, wherein at least one of the radial position (r) of the support (40) relative to the reference axis (51) and the orientation (a, b) of the support (40) is adjustable.

13. The measuring device (1) according to claim 12, further comprising a controller (6) that operably adjusts at least one of the radial position (r) of the support (40) relative to the reference axis (51) and the orientation (a, b) of the support (40) relative to the reference axis (51) based on at least one of the eccentricity (D) and tilt (T) of the measuring surface (11).

14. The measuring device (1) according to any one of claims 11 to 13 further includes a measuring head controller (66) operable to move or align the measuring head (60) relative to the support (40), wherein, in order to measure the position of at least the first measuring point (21, 31), the second measuring point (22, 32), and the third measuring point (23, 33) of the measuring surface (11) facing away from the measuring head (60), the measuring head controller (66) is configured to determine at least a first target point (26, 36) on one of the first surface (20) and the second surface (30) of the optical element (10) opposite to the measuring surface (11) and facing the measuring head (60). The first target point (26, 36), the second target point (27, 37), and the third target point (28, 38) are associated with one of the first measurement point (21, 31), the second measurement point (22, 32), and the third measurement point (23, 33), such that the measurement beam (61) entering the medium of the optical element (10) at the first target point (26, 36), the second target point (27, 37), and the third target point (28, 38) is retroreflected internally at the first measurement point (21, 31), the second measurement point (22, 32), and the third measurement point (23, 33), respectively.

15. A measuring device (1) for measuring the thickness of an optical element (10), the measuring device comprising: - Light source (2), which is configured to emit a measurement beam (61); - Support (40), the support (40) is used to fix the optical element (10); - A measuring head (60), which is connected to the light source (2), and the measuring head (60) is configured to: - Align the measurement beam (61) along or parallel to the optical axis (c) of the optical element (10); - Focus the measurement beam (61) along or parallel to the optical axis (c) and guide the measurement beam (61); - Move the focused measurement beam (61) along or parallel to the optical axis (c); The measuring head is configured to detect a portion of the reflected measuring beam and to detect the overlap between the focal region (68) of the focused measuring beam (61) and the first surface (20) and the second surface (30) of the optical element (10), wherein the second surface (30) is opposite to the first surface (20); and The measuring head is configured to detect the maximum value of the intensity of the reflected beam when the focused measuring beam (61) coincides with the first surface (20) and the second surface (30), respectively.

16. The measuring device (1) according to claim 15, wherein the measuring head is further configured to detect the overlap between the focused area of ​​the focused measuring beam (61) and the first surface (20) and / or the second surface (30) of the optical element (10) during the movement or scanning of the focused measuring beam along or parallel to the optical axis (c).

17. The measuring device (1) according to claim 15 or 16, wherein the measuring head is further configured to normalize or compensate the distance between a first position of the measuring head (60) along or parallel to the optical axis (c) and a second position of the measuring head (60) along or parallel to the optical axis (c) according to the refractive index of the medium of the optical element (10).

18. A computer program comprising instructions that, when executed by a processor (8, 9) of a measuring device (1) according to any one of claims 11 to 14, cause the processor (8, 9) to perform the steps of the method according to any one of claims 1 to 8, or when executed by a processor (8, 9) of a measuring device (1) according to any one of claims 15 to 17, cause the processor (8, 9) to perform the steps of the method according to claim 9 or 10.

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