Interference measurement device

Abstract

To provide an interference measurement device and a measurement method for measuring a surface or profile of an optical element.SOLUTION: A measurement method includes the steps of: forming at least three measurement points on one of measurement surfaces (11) of an optical element (10); measuring each measurement point's position by guiding a measurement beam (61) from a measurement head (60) to the measurement point and detecting measurement beam portions (62) reflected at each measurement point; determining at least one of a decentration and an inclination of the measurement surface (11) relative to a reference axis (51) in accordance with at least three measurement positions.SELECTED DRAWING: Figure 7

Description

[Technical Field]

[0001] The present disclosure relates to the field of interferometric measuring devices, and in particular to fiber-implemented interferometric measuring devices for measuring the surface, distance, and / or profile of an object by reflection of electromagnetic radiation from the surface of the object. [Background technology]

[0002] Quality control and precision measurement of optical surfaces of optical elements such as lenses is becoming increasingly important, especially for mass-produced optical elements. This may be particularly true for optical elements of fairly small size that can be used, for example, in miniature camera, imaging, and / or display systems.

[0003] For example, Patent Document 1 discloses an apparatus for measuring at least one surface section of an object mounted on a carrier, the apparatus including a reference object fixable relative to the carrier and a holder movable in at least a first direction relative to the reference object.

[0004] A reference body and a distance sensor are mounted rotatably relative to each other and are disposed on the holder. The distance measuring device is configured to determine a first distance to a first point on the surface of the object and a second distance to a second point on the reference body corresponding to the first point on the object. To this end, the distance measuring device includes first and second distance sensors, one of which faces toward the object and the other of which faces toward the reference body. Such an apparatus allows for highly precise and non-contact optical probing or scanning of the surface of an object.

[0005] For thickness measurements, particularly for measuring the thickness profile of an object and determining the wedge or slope of an optical surface, U.S. Patent No. 5,629,299 discloses another device that uses an object holder having an upper surface and a lower surface, which is selectively adapted to be positioned on a carrier in a first or second orientation. To measure the thickness of an object and correlate the surface profiles on both sides of the object, the first or upper surface of the object must be measured or scanned with the object holder positioned in the first orientation. The object must then be measured or scanned with the object holder positioned in the second orientation.

[0006] Positioning the object holder relative to the distance measuring device and reorienting the object holder with the object attached may require manual adjustment and / or calibration of each measuring device, which may be quite complex and time-consuming.

[0007] Such calibration and adjustment can be particularly important, for example, when the object to be measured and its respective mount are mounted on a rotating measurement stage that undergoes rotation during scanning of the optical element or the surface of the object. Typically, such measurement schemes require that the optical axis of the optical element to be measured be aligned with the rotation axis of the measurement stage.

[0008] In particular, in the case of surface or distance measurements based on interferometry, the distance between the measuring head and the surface of the optical element to be measured must remain within a predetermined measurement range. In the case of measurement methods based on optical interference, the measurement range for each distance measurement, and therefore the clear area, is given and is determined by the wavelength of the measurement beam. For example, by simultaneously using multiple measurement beams with different wavelengths, the measurement range, and therefore the clear area, can be extended. However, the rule that increases the spatial resolution of the measurement, respectively, decreases the measurement range, and therefore the clear area, applies. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] DE102011011065B4 [Patent Document 2] U.S. Patent Application No. 2017 / 0082521(A1) Summary of the Invention [Problem to be solved by the invention]

[0010] It is therefore an object of the present disclosure to provide an improved method for measuring surfaces and to provide an improved measurement device, for example, an interferometric measurement device for measuring the surface or profile of an optical element. The measurement device and method should provide effective and rapid measurement, as well as calibration and adjustment of the interferometric measurement device. Furthermore, the method and measurement device should provide fairly accurate and rapid measurement of the decenter, thickness, and / or tilt of at least one or both surfaces of the optical element without the need to modify the bearing or mounting of the optical element relative to the measurement device.

[0011] Another object of the present disclosure is to provide a respective computer program for carrying out such a measurement method and for controlling a respective interference measurement device. [Means for solving the problem]

[0012] 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 provided on opposite sides of the optical element. The first surface can be a top surface of the optical element, and the second surface can be a bottom surface. The optical element can be any type of optical lens.

[0013] In a first step, the method includes defining at least a first measurement point, a second measurement point, and a third measurement point on a measurement surface of the optical element. The measurement surface is one of the first surface and the second surface. The measurement points are predetermined measurement points. The measurement points can be selected and / or defined according to various criteria. In the case of this method of measuring a surface or profile of an optical element, the first, second, and third measurement points, respectively, are defined on the measurement surface and remain at least substantially fixed during the method of measuring the surface or profile of the optical element.

[0014] Generally, the shape and / or profile of the optical element is known with at least a minimum accuracy. To that extent, the surface and / or profile of the optical element is quantitatively known. In this method, the surface and / or profile must be precisely measured to a maximum accuracy, which is naturally higher than the known minimum accuracy. The minimum accuracy can be provided by the manufacturer of the optical element. The maximum accuracy can reveal manufacturing tolerances and / or defects in the surface or profile of the optical element. In typical cases, the minimum accuracy is in the micrometer range. The maximum accuracy 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.

[0015] The method serves to provide quality control and is therefore configured to measure the surface and / or profile of an optical element with maximum precision.

[0016] After defining at least first, second, and third measurement points on the measurement surface, a first position of the first measurement point is measured by directing a measurement beam from a measurement head to the first measurement point. A reflected, typically retro-reflected, portion of the measurement beam is detected at the first measurement point. 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 laser is typically used. An optical measurement procedure is applied, which may include a runtime analysis and / or an analysis of the relative phase between the reflected measurement beam portion and a reference beam.

[0017] Typically, the measurement head is configured to direct the measurement beam to a first measurement point and detect a portion of the measurement beam reflected at the measurement point. Typically, the measurement head is aligned along a surface normal of the measurement surface at the first measurement point or a region around the first measurement point. Typically, the measurement beam is directed to the first measurement point perpendicular to the measurement surface. Thus, the portion of the measurement beam reflected at the first measurement point re-enters the measurement head.

[0018] The deviation of the measurement beam from the surface normal to the first measurement point is tolerable up to a predetermined tolerance angle of the measurement head, typically a deviation of 15°, 10° or 5° or less from the surface normal at the first measurement point.

[0019] Then, at least a second position of the second measurement point and a third position of the third measurement point are subsequently measured. The measurement of the first and second measurement points is performed in the same manner as the measurement of the first measurement point described above. Here, the measurement beam is directed to the second measurement point and the respective measurement beam portion reflected at the second measurement point is detected, typically by a measurement head.

[0020] Similarly, the measurement beam is directed to at least a third measurement point, and respective measurement beam portions reflected at the third measurement point are detected, typically by a measurement head.

[0021] At least one of an eccentricity and a tilt of the measurement surface relative to the reference axis is determined based on the at least first position, the at least second position, and the at least third position.

[0022] The reference axis can be defined by a measurement stage of the measurement device, and the optical element can be located on the measurement stage for the surface and / or profile measurement. The reference axis can be a rotation axis about which the optical element can be rotated during a surface scan or a profile scan to determine at least one of the eccentricity and tilt of the measurement surface.

[0023] In principle, the at least first position, the at least second position and the at least third position can be compared with respective reference positions where the at least first, second and third measurement points would be located when the optical element is aligned with a reference axis, e.g. in the case of a tilt angle of 0 and / or a decentering of 0.

[0024] Typically, the first, second, and third positions of the actually measured measurement point can be numerically analyzed and fitted to a mathematical model having the eccentricity and tilt of the measurement surface as variables. By numerical calculation, for example, by fitting the first, second, and third positions to a first, second, and third reference position, respectively, the eccentricity and / or tilt of the measurement surface relative to a reference axis or respective reference planes defined, for example, by the first, second, and third reference points, can be determined.

[0025] Generally, a measured surface or a measured profile can be derived based on at least the first, second, and third measurement points. The measured surface or profile can be compared or fitted to a mathematical model of a respective reference surface or reference profile.

[0026] In this manner, the eccentricity and tilt of the measurement surface can be qualitatively and / or quantitatively determined simply by measuring the first, second, and third positions of the measurement surface.

[0027] Generally, the number of measurement points is determined by the type of measurement surface. Generally, the first, second, and third measurement points cannot be located on a common straight line. A line can be drawn through the first and second measurement points, but the third measurement point must be offset from such a line. If the measurement surface is spherical, the measurement surface can be unambiguously characterized by the first, second, and third measurement points.

[0028] If the measurement surface is aspheric, at least five measurement points must be defined and measured. If the optical element includes a so-called free-form surface as the measurement surface, at least six measurement points are required, which must be defined and subsequently measured by the above-mentioned method in order to unambiguously determine or calculate the decentering and / or tilt of the measurement surface. The number of measurement points depends on the number of degrees of freedom required to define the respective measurement surface.

[0029] In a typical example, the eccentricity and tilt determined by the above-described methods can be directly used to adjust and / or align an optical element for subsequent high-precision surface or profile measurements with one and the same measuring device. In this way, the above-described methods for determining at least one of the eccentricity and tilt of the measurement surface of an optical element can be directly used to align the optical element with respect to the measuring head of the measuring device.

[0030] By determining at least one of the eccentricity and tilt, the method can provide quantitative feedback regarding the direction and degree of eccentricity and the direction and degree of tilt. By precisely determining at least one of the eccentricity and tilt of the measurement surface, it is possible to provide fully automated or even partially automated adjustment or calibration of the optical element relative to the measurement head or reference axis. This can reduce the expenditure and effort for precisely aligning and calibrating optical elements for performing high-precision surface and / or profile measurements with a measurement device, for example, an interferometric measurement device.

[0031] According to another example, measuring at least one of the first position, the second position, and the third position includes focusing a measurement beam on at least one of the first measurement point, the second measurement point, and the third measurement point, and detecting a respective measurement beam portion retroreflected at at least one of the first measurement point, the second measurement point, and the third measurement point, respectively. By focusing the measurement beam on the first measurement point, an acceptance angle of the measurement head for the reflected measurement beam portion can be increased.

[0032] Furthermore, focusing the measurement beam at the respective measurement point increases the amount of light reflected at the respective measurement point, which can increase the signal-to-noise ratio for the detection of the reflected measurement beam portion.

[0033] According to a further example, one of the first and second surfaces of the optical element faces toward the measurement head. The other of the first and second surfaces faces away from the measurement head. This other surface can face toward a measurement stage on which the optical element is mechanically supported. Here, the second surface, which can face away from the measurement head toward the measurement stage, is the measurement surface. The first position, the second position, and the third position are measured by directing a measurement beam to a first target point located on one of the first and second surfaces facing toward the measurement head. Here, the measurement beam propagates through the medium of the optical element and strikes the first measurement point. The measurement beam is at least partially reflected at the first measurement point. Typically, the measurement beam is retroreflected through the medium toward the first target point and propagates from the first target point back to the measurement head.

[0034] Similarly, the method also includes measuring the second and third measurement points through the medium of the optical element. To measure the position of a second measurement point, the measurement beam is directed to a second target point arranged on one of the first and second surfaces facing the measurement head. Similarly, to measure the position of a third measurement point, the measurement beam is directed to a third target point on the one of the first and second surfaces facing the measurement head.

[0035] The first target point directly correlates to the first measurement point, the second target point directly correlates to the second measurement point, the third target point directly correlates to the third measurement point, etc. If more than two measurement points are provided, a respective number of target points are also provided on the surface opposite the measurement surface.

[0036] Typically, the measurement points are provided on a common surface of the first and second surfaces, and the target points are provided on a common surface of the first and second surfaces of the optical element, with the target points always being provided on the surface opposite to the surface on which the measurement points are located.

[0037] For example, by directing the measurement beam to at least first, second and third target points on the first surface, the position of the measurement point on the second surface can be measured and thus determined, which has the advantage that measurement points provided on a measurement surface facing away from the measurement head can be measured directly without having to turn the optical elements on the measurement stage, for example towards the measurement head.

[0038] This method for measuring the surface and / or profile of an optical element can be used to measure surfaces facing toward the measurement head as well as surfaces facing away from the measurement head. In this manner, measurements can be provided through the medium of the optical element. This allows both the first and second surfaces to be subsequently measured for decentering and / or tilt relative to a reference axis. Such subsequent measurements can be performed in a common coordinate or reference system. Thus, the tilt and / or decentering of the first surface can be directly correlated to the tilt and / or decentering of the second surface.

[0039] Thus, in a first measurement procedure, the eccentricity and tilt of the first surface of the optical element can be measured by measuring at least first, second, and third positions of first, second, and third measurement points located on the first surface of the optical element. The measurement surface can then be switched to the second surface without moving the optical element relative to the measurement stage. During a second measurement procedure, at least one of the eccentricity and tilt of the second surface can then be determined by directing a measurement beam to at least first, second, and third target points on the first surface and detecting respective reflected beam portions from the first, second, and third measurement points located 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.

[0040] Generally, the decenter and tilt of a first measurement surface relative to a reference axis can be determined. Because the optical element is not moved relative to the measurement stage between subsequent measurement procedures of the first and second surfaces, at least one of the decenter and tilt of a second surface of the optical element relative to the same reference axis can be determined. In this way, at least one of the decenter and tilt of the first surface of the optical element can be directly correlated to at least one of the decenter and tilt of the second surface of the optical element, respectively.

[0041] For the reciprocal mapping or assignment of the eccentricity and / or tilt of the first surface to at least one of the eccentricity and tilt of the second surface, the optical element can be stationary on the measurement stage, so that there is no need to change the orientation of the optical element between a first measurement procedure in which the first surface is the measurement surface and a second measurement procedure in which the second surface is the measurement surface.

[0042] According to a further example, the positions of the first, second, and third target points on the first surface and the second surface facing the measurement head are determined based on at least one of the refractive index of the medium of the optical element, the angle of incidence of the measurement beam with respect to the first, second, or third target point, and the local surface profile of at least one of the first surface and the second surface in the area of ​​the first, second, or third measurement point and / or in the area of ​​the first, second, or third target point.

[0043] Furthermore, the reciprocal mapping or assignment of the first target point to the first measurement point can also be determined based on the thickness or thickness profile of the optical element. This can also depend on the distance between the first surface and the second surface in the region of the first measurement point and the first target point. The same is valid for the reciprocal mapping and assignment of the second measurement point to the second target point and the third measurement point to the third target point.

[0044] Typically, at least the first, second and third measurement points are defined on a measurement surface, which coincides with a second surface, for example the lower surface of the measuring device facing towards the measurement table, with the corresponding at least first, second and third target points being located on a first surface opposite the second surface.

[0045] The method generally provides for measuring the position of a first measurement point. To this end, the general structure and shape and refractive index of the optical element are known to at least a predetermined minimum accuracy. Based on the local profile and shape of the optical element, the position of a first target point on the first surface is determined and / or calculated. To measure the position of the first measurement point, a measurement beam that is refracted at the first target point must be reflected, e.g., retroreflected, at the first measurement point.

[0046] In order to have effective retroreflection at the first measurement point through the optical medium of the optical element, the reflected portion of the measurement beam propagating from the first target point towards the first measurement point must strike the second surface substantially perpendicularly, so that the reflected beam portion of the measurement beam propagating from the first measurement point back towards the first target point spatially overlaps the measurement beam.

[0047] Because the refractive index of the material of the optical element differs from that of the surrounding air, the refraction of the measurement beam at the first target point must also be taken into account. The degree of refraction strongly depends on the surface profile in the area of ​​the first target point and the refractive index of the medium of the optical element. In some cases, even the radial position of the measurement head is predetermined and / or fixed. For a given radial position of the measurement head relative to the reference axis, only one distinct pair of target points and angles of incidence can always be provided. The measurement beam that strikes 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 is retroreflected at the first measurement point.

[0048] Since the overall shape and profile of the optical element is known with at least the minimum required accuracy and the refractive index of the medium of the optical element is known, it is always possible to calculate and / or determine, for each given measurement point of at least the first, second and third measurement points, one distinct first, second and third target point, respectively, on the opposite surface of the optical element.

[0049] Measuring the positions of at least first, second, and third measurement points on the measurement surface facing away from the optical head can also include propagating the measurement beam and the reflected beam portions through a medium of the optical element having a known refractive index. Further, for the position measurement, an optical beam path along which the measurement beam and the reflected beam portions propagate from the optical head through the medium toward the measurement points and back to the optical head is taken into account.

[0050] According to a further example, a first measurement path and a second measurement path are defined on the measurement surface. This can apply to both the first surface and the second surface, with the first and / or second surface being selected as the measurement surface, respectively. At least two of the first, second, and third measurement points are located on the first measurement path. At least one of the first, second, and third measurement points is located on the second measurement path.

[0051] By defining the measurement paths, a large number of respective measurement points located on the respective measurement paths can be defined. In this way, the accuracy of determining at least one of the eccentricity and tilt of the measurement surface can be improved. The more measurement points that are actually measured, the more accurately the calculation or determination of at least one of the eccentricity and tilt of the respective measurement surface can be performed.

[0052] In some examples, the optical element has radial symmetry. The optical element can include an optical axis. For methods of measuring a surface and / or profile of an optical element, it is particularly beneficial for at least two of the at least first, second, and third measurement points to be located at different radial distances to the optical axis of the optical element.

[0053] Thus, at least one of the at least three measurement points can be located a first radial distance from the optical axis of the optical element or a radial center point of the optical element, and another of the three measurement points can be located a second radial distance from the optical axis or optical center point of the optical element. In this manner, by using at least two measurement points located at different radial distances from the optical axis or central axis of the optical element, at least one of the decentering and tilting of the measurement surface can be determined based on measuring the positions of the at least first, second, and third measurement points.

[0054] According to a further example, at least one of the first and second measurement paths is a closed measurement path. This measurement path can include a circular or elliptical, e.g., oval, measurement path. A closed measurement path is particularly useful when the optical element is mounted on a rotating measurement stage, while the measurement head can be positioned at a variable radial distance to the rotation axis of the measurement stage.

[0055] In a closed measurement path, the measurement of at least the first, second, and third measurement points can start at any position on the measurement path. The measurement beam is guided along the measurement path by relative movement between the optical element and the measurement head until the measurement beam returns to its initial position on the measurement path.

[0056] According to another example, the first measurement path and the second measurement path are concentric with respect to an optical axis of the optical element or a reference axis defined, for example, by a measurement stage of the measuring device.

[0057] In the case of a circular or disk-shaped optical element, such as a lens, it is quite easy to define a concentric measurement path, for example a concentric measurement ring in which at least the first, second and third measurement points are arranged.

[0058] In general, the shape of the first and second measurement paths and the definition of their relative positions can be selected or calculated according to the particular shape or profile of the optical element.

[0059] According to another example, the optical element is attached to a mount. The mount is disposed on a measurement stage. The measurement stage is rotatable about an axis of rotation. At least one of a radial position and an orientation of the mount relative to the axis of rotation is adjustable and adjusted to minimize at least one of an eccentricity and a tilt of the optical element determined by the method.

[0060] In some examples, the axis of rotation of the measurement stage substantially coincides with or defines the reference axis, while in other examples, the reference axis is offset from the axis of rotation of the measurement stage.

[0061] The mount is typically radially movable relative to the axis of rotation. Furthermore, the mount is tiltable or pivotable about at least a tilt axis relative to the axis of rotation. Typically, the mount can be tiltable or pivotable about at least two tiltable axes, for example, located in a plane perpendicular to the axis of rotation or a reference axis.

[0062] The mount can be adjusted by at least one electrical drive to minimize at least one of the decentering and tilting of the optical element relative to a reference axis, in this example providing a type of automatic adjustment for the mount and, therefore, for the optical element attached to the mount.

[0063] According to another example, in a first measurement procedure, at least one of the eccentricity and tilt of one of the first surface and the second surface is determined, and in a second measurement procedure, at least one of the eccentricity and tilt of the other of the first surface and the second surface is determined. Between the first and second measurement procedures, the optical element and / or the mount remain stationary on the measurement table. The optical element, and therefore the mount for the optical element, can be stationary relative to the reference axis.

[0064] In this way, the eccentricity and / or tilt determined during the first measurement procedure can be directly correlated and / or mapped to the eccentricity and / or tilt obtained during the second measurement procedure.

[0065] According to another example, a method for measuring a surface and / or profile of an optical element includes measuring a thickness of the optical element along an optical axis of the optical element, wherein measuring the thickness includes aligning a measurement head along or parallel to the optical axis of the optical element, directing a measurement beam along or parallel to the optical axis, moving a focused measurement beam along or parallel to the optical axis, and finally detecting a coincidence of a focal zone of the focused measurement beam with a first surface and / or a second surface.

[0066] Generally, to measure the thickness of an optical element, a measurement beam is directed along a surface normal of a measurement point located on a first surface. A portion of the measurement beam is transmitted through the optical element and reflected from a corresponding second measurement point located on a second surface opposite the first surface. Here, the surface normal of the second point and the surface normal of the first point are substantially parallel. Generally, to measure the thickness, the focused measurement beam is moved or scanned longitudinally or axially, thus along the direction of the measurement beam.

[0067] This allows the measurement range of the measurement head to be reduced to a measurement range that is substantially shorter than the expected thickness of the optical element. When the focused measurement beam coincides with one of the first and second surfaces, the intensity of the reflected measurement beam portion is maximized. By subsequently moving the measurement beam, for example by scanning the measurement beam along or parallel to the optical axis through the medium of the optical element, further maxima in the reflected beam intensity can be detected. Those positions of the measurement head along the optical axis at which local maxima in the reflected beam portion can be detected indicate coincidence of the focused beam along the optical axis with one of the first and second surfaces.

[0068] The position of the measuring head along or parallel to the optical axis where the focused measurement beam coincides with the first surface is changed to the position along or parallel to the optical axis where the focused measurement beam coincides with the second surface. The first and second positions of the measuring head are compared with a second position of the measuring beam parallel to the optical axis. To calculate the thickness of the optical element, the first and second positions of the measuring head are compared or subtracted from each other, taking into account the refractive index of the medium of the optical element.

[0069] To distinguish between the reflected measurement beam portions reflected from the first and second surfaces of the optical element, the measurement range of the measurement head should be smaller than the thickness of the optical element. The coherence length of the measurement beam of the interferometric measuring device should be shorter or smaller than the distance between the first and second surfaces along the optical axis c. This can be achieved by using a suitable light source and an optical delay element 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.

[0070] The thickness measurement of the optical element can be performed without the above-described measurement of the first, second, and third positions of the first, second, and third measurement points, respectively, of the measurement surface. Nevertheless, the thickness measurement is particularly useful when the optical element is properly aligned and exhibits minimal decentering or tilt relative to the reference axis.

[0071] Thus, in another aspect, the present disclosure relates to a method for measuring the thickness of an optical element along an optical axis (c), the thickness measurement comprising aligning a measurement head along the optical axis of the optical element, directing a measurement beam from the measurement head along or parallel to the optical axis, and moving or scanning the focused measurement beam along or parallel to the optical axis.

[0072] The position and movement of the measurement head along the optical axis are tracked and controlled, and the ...

[0073] The distance between a first position of the measurement head where the focal area of ​​the focused measurement beam coincides with the first surface and a second position of the measurement head where the focal area of ​​the focused measurement beam coincides with the second surface directly indicates the thickness of the optical element in a direction along or parallel to the optical axis.

[0074] The distance between the first and second locations is further normalized or compensated to account for the refractive index of the medium of the optical element.

[0075] The thickness measurement of the optical element can be performed along the optical axis, and thus along the central axis of the optical element. The thickness measurement of the optical element can also be performed along a direction parallel to the optical axis. Here, the measurement beam is aligned parallel to the optical axis, but is radially or laterally offset from the optical axis. In this way, the thickness of the optical element can also be measured off-center from the optical axis. In general, the thickness of the optical element can be measured at a number of interrelated or corresponding points on the first and second surfaces, respectively. In this way, a thickness profile of the optical element can also be obtained.

[0076] Generally, the method for measuring thickness can be implemented without or despite the above-described measurements of at least the first, second, and third measurement points. The method can also be performed without a prior determination of the decenter or tilt of the measurement surface of the optical element.

[0077] In a further example of a method for measuring the surface or profile of an optical element, based on the determined tilt and / or decentering, the optical element is adjusted relative to the interferometric measuring device, in particular relative to the measuring head. In this way, the reference axis and thus the optical element relative to the measuring head are adjusted. Elimination or at least a substantial reduction of the original tilt and / or eccentricity can be achieved, after which high-precision topological measurements of the measurement surface can be made after elimination or at least reduction of at least one of the tilt and eccentricity.

[0078] In some examples, high-precision topology measurements can be made by rotating an optical element about a reference axis while moving a measurement head of an interferometric measurement device in transverse and longitudinal directions, where the longitudinal direction extends along or parallel to the rotation axis and the transverse direction extends perpendicular to the longitudinal direction, where the measurement surface of the optical element can be scanned or probed with multiple measurement points on the measurement surface.

[0079] Additionally, the measurement head can be directed at or aligned with each measurement point on the measurement surface of the optical element such that a measurement beam portion is reflected from the measurement point on the measurement surface towards the measurement head. Typically, the measurement beam is directed substantially parallel to or along the surface normal in the region of the measurement point on the measurement surface.

[0080] Suitable examples of high precision topology measurements are described in more detail in US Patent Application No. 2017 / 0082521(A1) or DE102011011065A1.

[0081] In the method for measuring a surface, preferably after determining at least one of tilt and decenter, and preferably after aligning the optical element with respect to a reference axis to eliminate or minimize the tilt and / or decenter, a high-precision topology measurement of the optical element can be performed, wherein the measurement surface subjected to the high-precision topology measurement can be one of a first surface and a second surface.

[0082] In some embodiments, the first surface can be a top surface of the optical element facing toward the measurement head. In some embodiments, the second surface can be a bottom surface of the optical element facing away from the measurement head. Here, high-precision topology measurements can be performed through the optical element. A measurement beam emanating from the measurement head can strike a first target point on the first measurement surface. The measurement beam can then propagate through the optical medium and reflect at the first measurement point on the second surface of the optical element. From here, typically, the measurement beam portion reflected at the first measurement point travels back through the optical element toward the first target point and again toward and / or into the measurement head.

[0083] Here, the optical phase of the returned measurement beam portion can be or is correlated and / or compared with the optical phase of a reference beam, for example generated by or within the measurement head. In this way, the topology of the first surface and / or the second surface can be measured with interferometric precision without having to reposition, reorient or move the optical element relative to a mount configured to hold it.

[0084] Thus, in a first step, at least one of the tilt and decenter of the first surface can be determined. In a second step, the tilt and decenter of the first surface can be substantially eliminated by respective adjustment of the optical element relative to the reference axis and / or the measuring head. In a further step, a high-precision topological measurement of the first surface can be performed.

[0085] Thereafter, or in an alternative sequence of the above steps, in at least one way, at least one of the tilt and decenter of the second surface of the optical element can also be determined by a respective measurement via the optical element, where in a further step the tilt and decenter of the second surface can be substantially eliminated by a respective adjustment of the optical element relative to the respective reference axis and / or measuring head.

[0086] In a further step, a high-precision topological measurement of a second surface can be performed by propagating the measurement beam through the optical element. Thus, by scanning from the first surface through the optical element, a high-precision measurement of the second surface of the optical element can be provided. In fact, a high-precision topological measurement of the first surface and the second surface located on the opposite side can be performed without changing the orientation of the optical element relative to the measurement head.

[0087] In another aspect, the present disclosure relates to a measurement device for measuring the surface and / or profile of an optical element. The measurement device can be implemented as a non-contact optical measurement device configured to scan the respective surface of the optical element. In some examples, the measurement device is an interferometric measurement device configured to direct a measurement beam to the surface of the optical element and detect beam portions reflected from the surface of the optical element.

[0088] The measurement device includes a light source configured to generate and direct a measurement beam to the measurement surface of the optical element. The measurement device further includes a mount for securing the optical element. The measurement device further includes a measurement head connected to the light source. The measurement head is configured to direct the measurement beam to the measurement surface of the optical element. The measurement head can be further configured to receive a measurement beam portion reflected from the measurement surface.

[0089] The measurement head is further movable relative to the mount to direct the measurement beam to at least a first predetermined measurement point, at least a second predetermined measurement point, and at least a third predetermined measurement point on the measurement surface.

[0090] The measurement device further includes a detector connected to the measurement head, the detector configured to detect respective measurement beam portions reflected at the at least first measurement point, the at least second measurement point, and the at least third measurement point.

[0091] The measurement device further includes a signal analyzer connected to the detector, the signal analyzer configured to determine a first position of the first measurement point, a second position of the second measurement point, and a third position of at least a third measurement point. The signal analyzer is further configured to determine at least one of eccentricity and tilt of the measurement surface relative to a reference axis, based on at least the first position, the second position, and the third position of the first, second, and third measurement points, respectively, of the measurement surface.

[0092] Typically, the measurement device is specifically configured to perform the above-described method of measuring the surface and / or profile of an optical element, and insofar as all configurations, examples, and operations described above in relation to the method apply equally to the measurement device, and vice versa.

[0093] To measure or determine at least one of the eccentricity and tilt of the measurement surface, the signal analyzer is configured to perform a numerical fitting procedure based on the measured first, second, and third positions of the first, second, and third measurement points, respectively. The measured positions are compared to predetermined positions of an optical element whose profile and geometry are known to at least a minimum accuracy. Based on the numerical fitting of the measured first, second, and third positions to a predetermined reference geometry of the optical element, at least one of the eccentricity and tilt of each measurement surface relative to a reference axis can be determined.

[0094] The determination of at least one of the eccentricity and tilt of the measurement surface includes and / or provides a degree of eccentricity and / or tilt relative to the reference axis. In this manner, the at least one of the eccentricity and tilt determined by the signal analyzer can be further used to determine the optical position of the measurement surface relative to the reference axis. The optical element can be appropriately aligned or adjusted. This adjustment or calibration can be done manually or automatically. When done manually, the entire procedure of measuring the first, second, and third measurement points and deriving the first, second, and third positions, respectively, can be repeated one or more times to repeatedly obtain the decenter and tilt of each of the measurement surfaces of the optical element.

[0095] In some examples, adjustment of the optical element, e.g., by reducing the decenter and / or tilt to a minimum, and thus alignment of the measurement surface of the optical element toward the reference axis, can also be performed automatically and / or deterministically. By precisely measuring the degree or size of the decenter and / or tilt, the interferometric measurement device, e.g., a controller of the measurement device, can automatically adjust the position and / or orientation of the mount of the optical element to eliminate or at least reduce the decenter and / or tilt of the measurement surface relative to the reference axis.

[0096] According to a further example, the mount is disposed on a rotatable measurement stage, and thus on a rotating measurement stage. The rotating measurement stage can define a reference axis. At least one of the radial position of the mount and the orientation of the mount relative to the reference axis is adjustable. Adjustment of the radial position of the mount and / or adjustment of the orientation of the mount can be performed manually. In some examples, the rotating measurement stage and / or the mount can include at least one or several electromechanical actuators, which can automatically adjust the radial position and / or orientation of the mount relative to the reference axis. In this way, fairly precise and rapid alignment of the mount, and thus the optical element attached to the mount, can be provided.

[0097] According to another example, the measurement device includes a controller operable to adjust at least one of the radial position of the mount and the orientation of the mount relative to a reference axis based on at least one of the eccentricity and tilt of the measurement surface, wherein the controller is particularly configured and adapted to control the movement or actuation of at least one of the actuators of the rotating measurement stage or the respective mount to move or orient the mount and / or optical element to a predetermined position and / or orientation relative to the reference axis. In this manner, semi-automated measurement-based eccentricity and tilt adjustments of the optical element relative to the reference axis can be provided.

[0098] Typically, in a further example, operation of the controller during adjustment of at least one of the mount's radial position and orientation relative to a reference axis can be monitored or controlled by the measurement head. Thus, during adjustment of at least one of the mount's radial position and orientation, the measurement head can be used to control the position of at least one of the mount and the optical element attached to the mount. In this manner, a feedback loop can be provided to provide further control of the controller during adjustment or calibration of the mount.

[0099] In practice, this can provide fairly precise and rapid alignment and / or positioning of the mount and / or optical element relative to the reference axis. This is particularly useful for reducing cycle times or clock cycles for sequentially measuring the surfaces or profiles of multiple optical elements. Semi-automated alignment and adjustment of the mount can provide subsequent high-precision measurements of the profile and / or surface of the optical element.

[0100] Furthermore, the reduced cycle time or clock cycle for adjustment is beneficial for using the measurement device for quality control of mass-produced optical elements.

[0101] According to another example, the measurement device further includes a measurement head controller operable to move and / or align the measurement head relative to the mount and / or the reference axis. To measure distances to at least a first measurement point, at least a second measurement point, and at least a third measurement point on the measurement surface facing away from the measurement head, the measurement head controller is configured to determine at least a first target point, a second target point, and a third target point on one of a first surface and a second surface of the optical element positioned opposite the measurement surface and facing toward the measurement head.

[0102] Each of the first, second, and third target points correlates to one of the first, second, and third measurement points, such that measurement beams entering the medium of the optical element at the first target point, the second target point, and the third target point are retroreflected internally at the first measurement point, the second measurement point, and the third measurement point, respectively.

[0103] Typically, the first target point is directly correlated to the first measurement point, the second target point is correlated to the second measurement point, and the at least third target point is correlated to the third measurement point. After determining the first target point, the second target point, and the at least third target point, the measurement head controller is configured to steer or orient the measurement head such that measurement beams emanating from the measurement head are directed to the first target point, the second target point, and the at least third target point, respectively, to provide measurements of the positions of the correlated first measurement point, the second measurement point, and the at least third measurement point.

[0104] In a further example, the measurement device is also configured to perform a high precision scan of the surface and / or profile of the optical element after aligning the optical element with respect to the reference axis.

[0105] During scanning or measuring the surface and / or profile of the optical element, the measurement head controller is moved relative to the optical element such that the measurement beam is directed substantially perpendicular to the measurement surface of the optical element.

[0106] According to another aspect, the present disclosure further relates to a computer program comprising instructions that, when executed by a processor of the above-mentioned measurement device, cause the processor to perform the steps of the above-mentioned method, and to that extent, all features, operations, and advantages described above in relation to the method and measurement device for surface and / or profile measurement of an optical element apply equally to the computer program, and vice versa.

[0107] The computer program may be implemented in a processor of a signal analyzer of the measurement device and / or in a processor of a measurement head controller. The computer program may be a distributed computer program. Some components of the computer program may be implemented or deployed in the processor of the signal analyzer. Other components of the computer program may be implemented or deployed in the processor of the measurement head controller.

[0108] In the following, a number of examples of methods for measuring the surface and / or profile of an optical element and interferometric measuring devices for measuring such a surface or profile will be described in more detail with reference to the drawings. [Brief explanation of the drawings]

[0109] [Figure 1] 1A-1C are diagrams illustrating schematically measuring the surface or profile of an optical element to determine the decenter of the optical element relative to a reference axis. [Figure 2] FIG. 2 is a diagram of the measured distances using the arrangement of FIG. 1. [Figure 3] 2 shows a further example similar to FIG. 1 in which the optical element is decentered and tilted compared to a reference axis. [Figure 4] FIG. 4 is a diagram of the measurement distances of the arrangement according to FIG. 3; [Figure 5] 1 is a side view of an example of measuring at least first, second, and third measurement points on a measurement surface of an optical element. [Figure 6] FIG. 6 is a top view showing the example of FIG. 5. [Figure 7] FIG. 10 is a side view showing a further example in which the positions of at least three measurement points provided on the bottom surface of the optical element are measured from above the optical element. [Figure 8] FIG. 8 is a bottom view showing the example of FIG. 7. [Figure 9] 1 illustrates an example of a method and a measuring device for measuring the thickness of an optical element. [Figure 10]FIG. 10 shows the relationship between signal strength and longitudinal displacement of the measurement head along the optical axis for the example of FIG. 9. [Figure 11] FIG. 10 shows a first configuration of the example of FIG. 9, in which the focal area of ​​the measurement beam coincides with the first surface. [Figure 12] FIG. 12 shows the example of FIG. 11, where the focal zone of the focused measurement beam coincides with the second surface. [Figure 13] FIG. 1 illustrates the configuration of a measurement device during a high precision surface or profile scan of a first surface. [Figure 14] 6 shows measurements of at least first, second and third measurement points on a second surface of an optical element, where the optical element is flipped over compared to the configuration of FIG. 5; [Figure 15] 15 shows a high precision surface or profile measurement of a second surface in the arrangement of FIG. 14. FIG. [Figure 16] 16A and 16B show diagrams illustrating the measurement of at least first, second and third measurement points on a first surface through a second surface in an optical element configuration according to FIGS. 14 and 15. [Figure 17] FIG. 2 is a block diagram of a measurement device. [Figure 18] FIG. 1 shows an example of an adjustable mount for an optical element arranged on a rotating measurement stage. [Figure 19] FIG. 2 is a detailed diagram of an example of an interference measuring device. [Figure 20] FIG. 20 is an enlarged view showing some details of the interferometric measuring device of FIG. 19. [Figure 21] 1 is a flow diagram of a method for measuring the surface or profile of an optical element. [Figure 22] FIG. 10 shows a schematic diagram of the numerical fitting procedure for obtaining the eccentricity and tilt of the measurement surface. [Figure 23] 10A and 10B are diagrams illustrating another example of measuring the thickness of an optical element in the outer edge region. DETAILED DESCRIPTION OF THE INVENTION

[0110] 1 shows a simplified example of a method for measuring at least one of the decentering D and tilt of an optical element 10 relative to a reference axis 51. A rotating measurement stage 50 is provided, which defines the reference axis 51. The reference axis 51 can coincide with a rotation axis 53. A mount 40 for the optical element 10, e.g., a lens, is provided on the measurement stage 50. The mount 40 is rotatable on the measurement stage 50 about the reference axis 51 or the rotation axis 53. An optical element 10 is further provided. The optical element 10 has a measurement surface 11. For the sake of simplicity, the optical element 10 is a cylindrical object and has a cylindrical side wall as the measurement surface 11.

[0111] The optical element 10 includes an optical axis c. The measurement device 1 includes at least one measurement head 60. The measurement head 60 is configured to measure the distance to the measurement surface 11 when the measurement stage 50 begins to rotate so as to move the mount 40 together with the optical element 10 about a reference axis 51 as the axis of rotation. The measurement stage 50 rotates the mount 40 and the optical element 10 relative to the reference axis 51 as the axis of rotation. As shown in FIG. 1 , the optical axis c of the optical element 10 is disposed at a radial offset D from the reference axis 51.

[0112] 1, the measuring head 60 can be moved from an upper position to a lower position 60′. The measuring head 60 then determines the distance to the measurement surface 11 at a first position 60 along the optical axis c and at a second position 60′ along the optical axis in order to measure the tilt T or decenter D of the optical element 10.

[0113] In FIG. 2, the distance D to the measurement surface 11 measured by the measurement head 60 is shown as a graph 70 over the rotation angle φ of the rotating measurement stage 50. The measurement distance of the measurement head 60' is represented by graph 70'. Because the geometric center, and therefore the optical axis c, is positioned at a radial offset D from the reference axis 51 or rotation axis 53, the distances according to graphs 70 and 70' vary over the rotation angle φ. Because the c-axis or central axis of the optical element 10 is parallel to the rotation axis 53 and the measurement surface 11 is cylindrical and includes a constant diameter along the length of the optical axis c, the slope and amplitude of graphs 70 and 70' are substantially equal. The amplitude of graphs 70 and 70' is a direct measure of the eccentricity D of the optical axis c and the reference axis 51.

[0114] This situation changes when the optical element is tilted or inclined relative to the reference axis 51 or the axis of rotation 53. Accordingly, the amplitude of the undulations of graph 70 measured at a position of measuring head 60 will be different from the undulations or amplitude of graph 70' measured at a further position of measuring head 60', as shown in Figures 3 and 4.

[0115] From the variations in amplitude and / or undulation of the graphs 70, 70', at least one of the decentering D and the tilt angle T of the optical axis c relative to the reference axis 51 can be determined.

[0116] Generally, when the geometry of the optical element 10 is known with at least a minimum accuracy and is clear from measurement points on the measurement surface 11 of the optical element 10, the tilt of the measurement surface 11 relative to the reference axis 51 as well as the eccentricity of the measurement surface can be obtained by numerical analysis. This can be obtained, for example, by matching the actually measured positions of the dedicated measurement points on the measurement surface 11 to known and predetermined reference points on the optical element 10.

[0117] The determination of eccentricity and / or tilt in a more comprehensive and schematic case is shown in FIGS.

[0118] Three dedicated and predetermined measurement points 21, 22, 23 are shown in the side view of Figure 5 and the top view of Figure 6. The measurement points 21, 22, 23 all have specific radial offsets from the central or optical axis c of the optical element 10. The optical element 10 includes a first surface 20 and an oppositely disposed second surface 30. In the embodiment shown, 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 from an optical medium 63 that is transparent to electromagnetic radiation.

[0119] Three dedicated measurement points 21, 22, 23 are provided on the first surface 20, which is now considered to be the measurement surface 11. These points 21, 22, 23 are fixed. These points 21, 22, 23 can be virtually defined on the measurement surface 11. These points 21, 22, 23 can be defined in a mathematical model of the optical element 10. These points 21, 22, 23 can be identified or defined taking into account the overall geometry, for example taking into account the circumferential boundaries of one of the first surface 20 and / or the second surface 30, respectively.

[0120] When the first surface 20, and therefore the measurement surface 11, is spherical, it is generally sufficient to define at least three measurement points 21, 22, 23 on the measurement surface 11. The measurement head 60 is then used to measure a first position of the first measurement point 21, a second position of the second measurement point 22, and a third position of the third measurement point 23.

[0121] Each position measurement is obtained by directing a measurement beam 61 from a measurement head 60 towards, for example, a first measurement point 21. A beam portion 62 of the measurement beam 61 that is reflected at the first measurement point 21 is captured and detected by the measurement head 60. The first position measurement can include measuring the distance between the measurement head 60 and the first measurement point 21.

[0122] When the measurement device 1 is implemented as an interferometric measurement device, the measurement head 60 can be configured to determine a path difference of the reflected measurement beam portion compared to a reference beam. A phase shift between the reflected measurement beam portion and the reference beam can indicate the distance between the measurement head 60 and the first measurement point 21. For example, the position and / or orientation of the measurement head 60 in the global coordinate system of the measurement device 1 is precisely known.

[0123] Therefore, by measuring the distance between the measuring head 60 and the first measurement point 21 it is possible to determine the position of the first measurement point 21 in the global coordinate system of the measuring device 1 .

[0124] Similarly, a second position of the second measurement point 22 and a third position of the third measurement point 23 can be obtained. To this end, the measurement head 60 undergoes a respective movement towards a position 60', as shown in Figure 5. In addition, the optical element 10 can undergo a rotation relative to the reference axis 51.

[0125] After measuring at least the first, second, and third positions of the at least first, second, and third measurement points 21, 22, and 23, the orientation and position of each measurement surface 11 relative to a reference surface of the optical element can be calculated and determined, typically by a numerical fitting operation. The numerical fitting operation is implemented by a computer program. Here, the program includes 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. Next, by fitting at least the first, second, and third positions actually measured by the measuring head 60, 60′ to a numerical model of the reference optical element or reference measurement surface, the decentering and / or tilt angle of the measurement surface 11 relative to the respective reference surface or reference axis can be numerically determined.

[0126] 22 visualizes the determination of the decentering D and tilt T of the measurement surface 11 based on a numerical fit to a reference surface 11′. Here, the axial position of the optical element 10 relative to the first, second, and third measurement points 21, 22, 23 on the measurement surface 11 is shown schematically. Based on the position measurements of the at least three measurement points 21, 22, 23, the overall shape and geometry of the optical element 10 is known to at least a minimum accuracy, so that the orientation and position of the measurement surface 11 can be characterized and determined by the numerical fitting procedure.

[0127] A reference surface 11' of the reference optical element 10' is further indicated by a dashed line. The reference optical element 10' represents the position and orientation of the optical element 10 when perfectly aligned for 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, 23 are numerically fitted with a mathematical model having the eccentricity D and the tilt angle T as variables. The measurement positions of the at least the first, second, and third measurement points 21, 22, 23 are numerically fitted to minimize deviations from the reference surface 11'. This results in a numerical determination of the respective tilt angles T and the respective radial eccentricities D.

[0128] In this way, by examining at least the first, second and third positions of the dedicated and predetermined first, second and third measurement points 21, 22, 23 on the measurement surface 11 of the optical element 10, the decentering D and / or tilt of the measurement surface 11 compared to the reference axis 51 is determined. For a spherical measurement surface 11, it is sufficient to use only three separated measurement points 21, 22, 23. If the measurement surface 11 comprises a non-spherical shape, at least five dedicated and predetermined measurement points on the measurement surface are required. In the case of a free-form surface of the measurement surface, at least six dedicated and / or predetermined measurement points 21, 22, 23 are required.

[0129] In this example, a first measurement path 25 and a second measurement path 29 can be provided on the measurement surface 11. In the example shown, the measurement paths 25, 29 are both closed measurement paths. The first measurement path 25 can include a circle or an ellipse on the measurement surface 11. The second measurement path 29 can also include a circle or an ellipse. As further shown, the first and second measurement points 21, 22 are located on the first measurement path 25. Only the third measurement point 23 is located on the second measurement path 29.

[0130] By selecting or defining at least first and second measurement paths 25, 29 on the measurement surface 11 and using a large number of measurement points on the at least two measurement paths 25, 29, the positions of a relatively large number of measurement points can be determined. The measurement head 60 can scan along the first measurement path 25 and / or the second measurement path 29, thus determining the positions of a respective number of measurement points on the measurement surface 11. Generally, the more measurement point positions are obtained, the more precisely the fitting procedure can be performed to determine at least one of the eccentricity and tilt of the measurement surface compared to the reference axis 51.

[0131] 7 and 8, as shown in a bottom view of optical element 10, second surface 30 of optical element 10 is defined as measurement surface 11, and measurement surface 11 is provided with first measurement point 31, second measurement point 32, and third measurement point 33. Again, first and second measurement points 31, 32 are located on first measurement path 35. Third measurement point 33 is located on second measurement path 39.

[0132] In the example shown, the first and second measurement paths 25, 35, 29, 39 are concentric. However, the first and second measurement paths 25, 35, 29, 39 can also be non-concentric or oblique to each other. It is also possible for the measurement paths 25, 29, 35, 39 to intersect each other.

[0133] To measure the positions of the first, second, and third measurement points 31, 32, and 33, the measurement head 60 is still positioned on the first surface 20 of the optical element. As shown in FIG. 7 , the position measurements of the first, second, and third measurement points 31, 32, and 33 are performed through the medium 63 of the optical element 10. To precisely 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, first, second, and third target points 26, 27, and 28 are defined on the opposite side, and thus on the first surface 20 of the optical element 10, respectively. The first target point 26 directly correlates to the first measurement point 31. The second target point 27 directly correlates to the second measurement point 32, and the third target point 28 directly correlates to the third measurement point 33.

[0134] Since measuring the positions of multiple measurement points 31, 32, 33, and therefore the distance between measurement head 60, 60′ and measurement points 31, 32, 33, involves propagating measurement beam 61 through medium 63, each target point 26, 27, 28 is calculated and / or determined such that measurement beam 61 directed toward first target point 26 is refracted at first target point 26, and refracted portion 61′ of the measurement beam is retroreflected at first measurement point 31 by second surface 30. Retroreflected measurement beam portion 62′ propagating through medium 63 is then refracted again at first surface 20 and re-enters measurement head 60 as reflected measurement beam portion 62 in the opposite direction to measurement beam 61.

[0135] The selection and determination of the target points 26, 27, 28 is based on the refractive index of the medium 63, the angle of incidence of the measurement beam 61 at the respective target points 26, 27, 28. Additionally, the selection and determination of the target points may also take into account the slope or surface profile of the measurement points 31, 32, 33 and / or the area of ​​the target points 26, 27, 28, as well as the thickness or profile of the optical element 10.

[0136] In some examples, the determination or calculation of target points 26, 27, 28 on the first surface, for example the surface facing the measurement head 60, may also take into account at least one of the position or orientation of the measurement head 60, 60'.

[0137] Furthermore, to determine the optical path length between the measurement head 60, 60' and the measurement points 31, 32, 33 on the surface 30 of the optical element 10 facing away from the measurement head 60, 60', the geometry of the optical path as well as the refractive index and path length of the beam propagating through the medium 63 are taken into account.

[0138] Since the refractive index of the medium 63 and the geometry and profile of the optical element 10 are known to a minimum accuracy, the positions of the measurement points 31, 32, 33 can be determined for a sufficiently precise determination of at least the eccentricity D and / or tilt of the respective measurement surface 11 relative to the reference axis 51.

[0139] Measuring a number of measurement points 21, 22, 23 on the measurement surface 11 coinciding with the first surface of the optical element 10 makes it possible to determine at least one of the eccentricity and tilt of the first surface 20 relative to a reference axis. This determination or measurement may constitute a first measurement procedure. Measuring the first, second, and third measurement points 31, 32, 33 on the second surface 30 of the optical element 10 constitutes a second measurement procedure. When switching from the first measurement procedure shown in FIG. 5 to the second measurement procedure shown in FIG. 7, the position of the optical element 10 on the mount 40 and / or measurement stage 50 may remain uncorrected.

[0140] To that extent, both sequential measurement procedures, one 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, are directly and therefore inherently correlated to 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 mount 40 and / or the optical element 10 to determine the eccentricity and tilt of surfaces 20, 30 located on opposite sides of the optical element 10.

[0141] Thus, direct mapping and assignment of geometric data and properties of surfaces 20, 30 located on opposite sides of the optical element 10 can be obtained without the need to reorient or flip the optical element 10 or the mount 40 between successive measurement procedures.

[0142] 9 to 12, the measurement of the thickness of the optical element 10 is shown diagrammatically. For this purpose, a measuring head 60 emitting a measuring beam 61 is aligned along the optical axis c of the optical element 10. This alignment can be performed after determining the decentering D and tilt T of the optical element 10 relative to the reference axis 51. The measuring beam 61 is focused along the optical axis c.

[0143] Next, the measurement head 60 can be moved so that the focused measurement beam 61 is aligned along the optical axis. As shown in Figure 10, when the focal zone 68 of the focused measurement beam 61 coincides with the first surface 20 of the optical element 10, the intensity or signal strength of the reflected beam portion detected and / or captured by the measurement head 60 is at a maximum M1, as shown in Figure 10. The maximum M1 is obtained when the measuring head 60 is in the first measuring position z1.

[0144] When the measurement head 60' is moved along the optical axis, for example from the position shown in Figure 11 towards the position shown in Figure 12, a focal zone 68 of the focal point of the measurement beam 61' approaches the second surface 30 of the optical element. A second maximum M2 of the captured and reflected measurement beam occurs when the focal zone 68' or focal point coincides with the second surface 30, i.e. when the focal zone 68 intersects the intersection of the optical axis c and the second surface 30. This maximum M2 is obtained when the measurement head 60' is in a second axial position z2.

[0145] The difference or distance between positions z1 and z2 directly indicates the thickness of optical element 10 along optical axis c. To precisely determine the thickness, the refractive index of medium 63 is also taken into account.

[0146] FIG. 23 shows another example of measuring the thickness of an optical element 10 off-axis from the optical axis or c-axis but parallel to the optical axis. Here, a measurement beam 61 is directed to an edge 72 of the optical element 10. The edge 72 is the radially outer edge of the optical element, e.g., an optical lens. The first surface 20 and the second surface 30 within the edge 72 extend substantially parallel to each other. The measurement beam 61 is directed parallel to the optical axis c to a first measurement point 74 on the first surface 20 within the edge 72. At least a portion 61′ of the measurement beam 61 is transmitted through the medium 63 and retroreflected at a second measurement point 75 located on the opposite side on the second surface 30 within the edge 72. From there, the reflected measurement beam portions 62′ and 62′ return toward and enter the measurement head 60.

[0147] As described above in connection with Figures 9 to 12, the position of the measurement head 60 is changed or moved along the propagation direction of the measurement beam, typically along or parallel to the optical axis c, in order to detect local maxima of the reflected light intensity at the first measurement point 74 and / or the second measurement point 75, respectively.

[0148] 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 above-described measurements can be applied to any region of an optical element, where the surface normal of a first point on a first surface runs substantially parallel to the surface normal of a second point on a second surface located on the opposite side. Typically, this requirement is usually realized in the region of the c-axis by optical lenses.

[0149] After determining the eccentricity D and / or tilt T of the measurement surface 11 relative to a reference axis 51, e.g., the axis of rotation 53 of the measurement stage 50, the measurement surface 11, e.g., the first surface 20, can be precisely measured by scanning the measurement head 60 over the measurement surface 11, as shown by multiple positions 60, 60′, and 60″ shown in FIG. 13 . This measurement of the surface closely matches the measurement procedures described in U.S. Patent Application No. 2017 / 0082521(A1) or DE102011011065A1, the entire contents of which are incorporated herein by reference.

[0150] The example shown in Figure 14 is somewhat equivalent to the example shown in Figure 5, except that the optical element 10 is flipped over so that the second surface 30 faces upwards towards the measurement head 60. Here, the measurement surface 11 coincides with the second surface 30, and first, second and third measurement points 31, 32, 33 located on the second surface 30 are measured and probed in a similar manner as described above with reference to Figure 5.

[0151] Again, after determining at least one of the tilt T and decenter D of the measurement surface 11 of the second surface 30, a high precision surface scan or profile scan of the second optical surface 30 can be performed, as described above in connection with FIG. 13.

[0152] 16, it is also possible and envisaged to define target points 36, 37, 38 on the second surface 30. In this way, measurement points 21, 22, 23 provided on the first surface 20, which here face away from the measurement head 60, can be measured. Thus, by defining corresponding first, second and third target points 36, 37, 38 on the upward-facing second surface 30, the positions of the first, second and third measurement points 21, 22, 23 on the first surface 20 can be measured through the medium 63 of the optical element. In this way, measurement accuracy can be improved.

[0153] The measurement procedure shown in Figure 5 allows for a direct determination of at least one of the eccentricity and tilt of the first surface 20. In the configuration of Figure 16, the definition of target points 36, 37, 38 on the oppositely disposed second surface 30 allows for a measurement of the eccentricity and / or tilt of the first surface 20 through the medium 63. The determination of the eccentricity and / or tilt of the first surface 20 obtained by the measurement procedure of Figure 5 can be compared and correlated with the measurement of the eccentricity and / or tilt obtained by the measurement procedure with the configuration of the optical element 10 shown in Figure 16. In this way, the eccentricity and tilt of both the first surface 20 and the second surface 30 can be measured in two different ways, thus improving the accuracy of the determination of the eccentricity and / or tilt of each surface 20, 30 of the optical element 10.

[0154] 17 shows an example of a measurement device 1 in block diagram form. The measurement device 1 includes a light source 2, an optical coupler 3, a detector 4, and a measurement head 60. The measurement head 60 is optically coupled to the light source 2 through an optical fiber 7. A measurement beam 61 generated by the light source 2 can be directed to the measurement head 60 via the optical coupler 3 and the optical fiber 7. When the measurement device 1 is implemented as an interferometric measurement device, the measurement beam 61 is split into a signal beam and a reference beam that are directed to the optical element 10. Some examples are described in more detail, for example, in DE102011011065B4 or at least one of US Patent Application No. 2017 / 0082521(A1).

[0155] The reference beam can be generated at a fiber exit face located within the measurement head 60. A measurement beam portion 62 reflected from the surfaces 20, 30 of the optical element 10 is captured by the measurement head 60 and propagates together with the reference beam within the optical fiber 7 towards the optical coupler 3. In a typical example, the optical coupler 3 includes an optical circulator.

[0156] Light propagating from measurement head 60 towards optical coupler 3 is redirected towards detector 4. Detector 4 includes a number of light-sensitive elements, such as an array or matrix of charge-coupled devices (CCDs), for detecting an interference pattern produced by interference between the reference beam and captured measurement beam portion 62 reflected from one of surfaces 20, 30 of optical element 10. Detector 4 is connected to signal analyzer 5 for resolving and / or determining the relative phase between the reflected signal beam and the reference beam.

[0157] Typically, the signal analyzer 5 includes a processor 8 to calculate or determine the relative phase, and therefore the optical path difference, between the reflected signal beam and the reference beam acquired and / or captured by the measurement head 60. Based on the optical path difference, the distance to a selected point on the surface 20, 30 of the optical element 10 can be determined. By further knowing the exact position of the measurement head relative to the global coordinate system of the measurement device 1, the position of each measurement point on the surface 20, 30 of the optical element 10 can be obtained.

[0158] As mentioned above, an optical element 10, for example in the form of a lens, is mounted on a mount 40. The mount 40 is rotatably supported on a rotating measurement stage 50. The measurement stage 50 defines a reference axis 51. The reference axis 51 can coincide with a rotation axis 53 defined by the measurement stage 50.

[0159] The measurement device 1 further includes a measurement head controller 66. The measurement head controller 66 includes at least a processor 9. The measurement head controller typically controls and manages the position and orientation of the measurement head 60. The signal analyzer 5 and the detector 4 can be implemented as integral parts of the controller 6 of the measurement device 1. In this manner, the measurement head controller 66 can also be implemented as a part of the controller 6, for example as an integral part. The processors 8, 9 shown here can also be integrated into a single processing unit of the controller 6.

[0160] The measurement head controller 66 can also be implemented as a separate controller. The controller 6 is configured to control or communicate with the measurement head controller 66. In this manner, 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 assign the measurement beam portions 62 captured from each measurement point to the respective measurement points on the surfaces 20, 30.

[0161] In some examples, the mount 40 can be controllable by the controller 6. The controller 6 can thus be configured to orient or move the mount 40, and thus the optical element 10, relative to the reference axis 51. In this manner, the controller 6 can be configured to automatically adjust the decentering and / or tilt of the optical element 10 relative to the reference axis 51. This example is described based on cylindrical coordinates. Because the mount 40 is rotatable relative to the rotation axis 53 and thus the reference axis 51, the mount 40 can be radially displaceable relative to the stationary measurement stage 50. The mount 40 can also be tiltable about 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 the rotation axis 53. The tilt axes a and b can be stationary relative to the mount 40. In some examples, the tilt axes a and b can be reconfigurable, and thus can vary in position and / or orientation.

[0162] 18 to 20, the mount 40 includes a base 41 that can be fixed to a measurement table 50. An intermediate member 42 is disposed and positioned on the upper surface of the base 41. An upper member 44 is provided on an upper surface 43 of the intermediate member 42. The intermediate member 42 is somewhat loosely fitted onto the upper surface of the base 41. The intermediate member 42 can frictionally engage with the upper surface of the base 41. The lower or bottom surface of the intermediate member 42 can be flat and can come into surface contact with the upper surface of the base 41, which has a complementary flat shape.

[0163] In this manner, the reference axis 51 or rotation axis 53 extends substantially perpendicular to the planar surfaces of the intermediate member 42 and the base 41, so that the intermediate member 42 is displaceable relative to the base 41 along the radial direction r. The intermediate member 42 is radially displaceable through an actuator 47. The actuator 47 may include a type of mechanical pulse generator or pulsing device configured to apply a radial force to the intermediate member 42. In this manner, the intermediate member 42 can undergo a radial shift relative to the base 41.

[0164] The upper surface 43 of the intermediate member 42 is dome-shaped. The lower surface 45 of the upper member 44 is a complementary dome-shaped surface. Thus, the upper surface 43 can include a concave shape, and the lower surface 45 of the upper member 44 can include a corresponding or complementary convex shape. The roles of the convex and concave upper and lower surfaces of the intermediate member 42 and the upper member 44 can also be reversed. Typically, the dome-shaped surfaces 43, 45 include convex and concave shapes in both directions transverse to the axis of rotation 51.

[0165] Another actuator 48 is further provided that is configured to selectively engage the top member 44. The actuator 48 may also include a pulse generator or pulsing device configured to repeatedly apply a kinetic force to the outer edge or side of the top member 44. In this manner, the corresponding dome-shaped surfaces 45, 43 allow the top member 44 to tilt relative to the intermediate member 42 as the dome-shaped surface 45 slides over the corresponding dome-shaped surface 43. The dome-shaped surfaces 43, 45 are frictionally engaged and maintain their orientation relative to one another even when subjected to rotation about the axis of rotation 51 under the influence of gravity.

[0166] After the eccentricity D and / or tilt T of the measurement surface 11 are determined, the controller 6 can be configured to adjust the alignment or positioning of the mount 40 relative to the reference axis 51. Accordingly, the controller 6 can control and activate the actuators 47, 48 to align and position the optical element 10 for the subsequent high-precision surface scanning process shown in FIG. 13 or FIG. 15 . During operation of the actuators 47, 48, the position and / or orientation of the optical element 10 can be monitored by the measurement head 60. In this manner, the distance or position measurements provided by the measurement head 60 during the movement or alignment of the mount 40 constitute a feedback loop. This is particularly beneficial for high-precision automated adjustment or calibration of the mount for high-precision surface scanning procedures.

[0167] A practical implementation of mount 40 is shown in Figure 20. As shown there, top member 44 is provided with chamfers 49 along its outer and / or upper edges. Actuator 48 is configured to apply momentum to top member 44 and is aligned with chamfer 49 so as to impinge or strike chamfer 49 substantially perpendicularly. A further actuator 47 is aligned horizontally and thus configured to apply radially directed momentum to intermediate member 42.

[0168] FIG. 19 shows an example of a measuring device 1. The measuring device 1 closely correlates with devices described and illustrated in more detail in, for example, DE102011011065B4 or US Patent Application No. 2017 / 0082521(A1). The measuring device 1 includes a stationary base 81. Upward-pointing legs 83 extend from opposite sides of the base 81. The upper ends of the legs 83 are connected by a cross member 84. The cross member 84 and the legs 83 form a frame 82 attached to the base 81. The rotating measuring stage 50 is disposed at the bottom of the base 81. The base 81 may further include an upward-pointing or upward-extending back surface 85. A holder 90 and a distance measuring device 92 are provided on the back surface 85. The holder 90 is movable in at least two longitudinal directions, for example, along the horizontal (x) and vertical (z) directions. The holder 90 may also be movable along a second horizontal y-direction relative to the rotation stage 50. The x and y directions may define a radial plane perpendicular to the rotation axis 53 or reference axis 51.

[0169] The holder 90 is further provided with a bearing 91. The bearing 91 is rotatably attached to the holder 90. Typically, the bearing 91 can include or define a rotation axis extending along the y-direction. A reference body 95 is further provided on the holder 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 mount 40 and, therefore, the optical element 10 disposed on the mount 40. The optical element 10 can be located on a support 46. The support 46 can be located on the upper member 44 of the mount 40. In some examples, the support 46 includes a hydraulic expansion chuck that enables the optical element 10 to be fixed and / or securely held.

[0170] At least a first reference sensor 88 facing the first reference surface 86 provides a measurement data. The position of the distance measuring device 92 in the global coordinate system of the vise 1 can be precisely determined. A reference surface 86 extends vertically, thus along the z-direction, and is attached to one of the upwardly facing legs 83. A second reference sensor 89 can face towards another reference surface 87 provided on the traverse object 84. The reference surface 87 extends horizontally, for example along the x-direction.

[0171] Reference sensors 88, 89 are located and fixed on holder 90. Reference sensors 88, 89 are configured to determine the position of holder 90 in the x-z plane. Both reference sensors 88, 89 can be implemented as distance sensors. Reference sensors 88, 89 are configured to determine the distance to respective calibrated reference surfaces 86, 87, respectively.

[0172] The distance measuring device 92 is rotatably mounted on the holder 90 and is pivotable about a rotation axis extending substantially along the y direction. The distance measuring device 92 includes a first distance sensor 93 facing towards the mount 40 and therefore towards the optical element 10. The distance measuring device 92 further includes a second distance sensor 94 facing towards a reference surface 96 of a reference body 95 fixed to the holder 90.

[0173] In the example shown, the first distance sensor 93 and the second distance sensor 94 extend in opposite directions, e.g., diametrically opposite directions. The second distance sensor 94 is configured to determine the distance from a reference surface 96 to the distance measuring device 92. In this way, any positional changes of the distance measuring device 92 that may occur due to rotation of the distance measuring device 92 relative to the holder 90 can be precisely compensated for and tracked.

[0174] The operation of the measurement device 1 and the multiple steps of the method for measuring the surface 20, 30 or profile of the optical element 10 are further explained in the flow chart of Figure 21. In a first step 100, a large number of measurement points 21, 22, 23, for example at least three, are defined on the measurement surface 11 of the optical element 10. Depending on the type of measurement surface 11, the total number of predetermined measurement points can vary. In a typical example, a relatively large number of measurement points 21, 22, 23 are defined and arranged on at least two measurement paths 25, 29, for example as shown in Figures 5 and 6.

[0175] Thereafter, in step 102, the measurement head 60 is moved relative to the optical element 10 to scan along the measurement paths 25, 29. At least the measurement device 1 is operated such that first, second and third positions of a number of measurement points 21, 22, 23 are obtained. Based on the position measurements obtained in step 102, in a subsequent step 104 at least one of the eccentricity D and the tilt T of the measurement surface 11 relative to the reference axis 51 is determined.

[0176] Based on the determined tilt T and / or decenter D, the optical element 10 is adjusted in step 106 .

[0177] The precise adjustment, and thus the elimination or substantial reduction of tilt T and / or eccentricity D, is particularly beneficial for the subsequent high-precision topology measurement of the measurement surface 11, which is carried out in step 108. The topology or surface measurement of the optical element 10 is typically carried out by rotating the optical element 10 by means of a rotating measurement stage 50 and scanning it over at least a portion of or the entirety of at least one of the measurement surfaces 20, 30, as shown, for example, schematically in Figure 13 or 15. During or for the high-precision topology measurement of the measurement surface, the measurement head 60 and the respective measurement beam 61 are particularly focused on the surface 20, 30 facing towards the measurement head 60.

[0178] After the topology measurement is performed, for example as shown in Figure 13, the measurement head 60 can be aligned along the optical axis c of the optical element 10 as shown in Figure 9. Thereafter, in step 110, the measurement head 60 is moved along the optical axis c as shown by Figures 9-12. The thickness can be measured.

[0179] To perform a thickness measurement, the measurement range of the measurement head should be smaller than the thickness of the optical element 10. With an interferometric measuring device, the coherence length of the measurement beam should be shorter or smaller than the distance between the first surface 20 and the second surface 30 along the optical axis c. This can be obtained by using a suitable light source 2 as well as 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 1. [Explanation of symbols]

[0180] 1. Measuring Device 2 light source 3 Optical Coupler 4. Detector 5 Signal analyzer 6 Controller 7. Optical Fiber 8 processors 9 processors 10 Optical elements 11 Measurement surface 20 surface 21 measurement points 22 measurement points 23 measurement points 25 Measurement Path 26 target point 27 Target point 28 target point 29 Measurement Path 30 surface 31 measurement points 32 measurement points 33 measurement points 35 Measurement Path 36 Target point 37 Target point 38 Target point 39 Measurement Path 40 Mount 41 Base 42 Intermediate parts 43 Domed Surface 44 Upper member 45 Domed Surface 46 Support 47 Actuator 48 Actuator 49 Chamfering 50 Measuring Table 51 Reference axis 53 Rotation axis 60 measuring heads 61 Measurement beam 62 Measurement beam section 63 Medium 66 Measuring head controller 68 Focal Area 70 graphs 72 Edge 74 measurement points 75 measurement points 81 base 82 slots 83 Legs 84 Crossings 85 Back side 86 Reference plane 87 Reference plane 88 Reference Sensor 89 Reference Sensor 90 Holder 91 Bearing part 92 Distance Measuring Devices 93 Distance Sensor 94 Distance Sensor 95 Reference Body 96 Reference plane

Claims

1. A method for measuring a surface (20, 30) or profile (P) of an optical element (10), the optical element (10) comprising a first surface (20) and a second surface (30) opposite the first surface (20), the method comprising: defining at least a first measurement point (21, 31), a second measurement point (22, 32), and a third measurement point (23, 33) on a measurement surface (11) of an optical element (10), the measurement surface (20, 30) being one of a first surface (20) and a second surface (30); measuring a first position of the first measurement point (21, 31) by directing a measurement beam (61) from a measurement head (60) to the first measurement point (21, 31) and detecting a measurement beam portion (62) reflected at the first measurement point (21, 31); thereafter, measuring at least a second position of the second measurement point (22, 32) and a third position of the third measurement point (23, 33) by directing a measurement beam (61) to the second measurement point (22, 32) and a third measurement point (23, 33), respectively, and detecting measurement beam portions (62) reflected at the second measurement point (22, 32) and the third measurement point (23, 33); and determining at least one of an eccentricity (D) and a tilt (T) of the measurement surface (11) relative to a reference axis (51) based on at least the first position, the second position, and the third position.

2. 2. The method of claim 1, wherein the step of measuring at least one of the first position, the second position, and the third position includes focusing a measurement beam (61) on at least one of the first measurement point (21, 31), the second measurement point (22, 32), and the third measurement point (23, 33), and detecting a respective measurement beam portion (62) retroreflected at at least one of the first measurement point (21, 31), the second measurement point (22, 32), and the third measurement point (23, 33), respectively.

3. 3. The method according to claim 1, wherein one of the first surface (20) and the second surface (30) of the optical element (10) faces towards the measurement head (60), and the other of the first surface (20) and the second surface (30) facing away from the measurement head (60) is the measurement surface (11), and the first position, the second position, and the third position are measured by directing a measurement beam (61) to first target points (26, 36), second target points (27, 37), and third target points (28, 38), which are located on one of the first surface (20) and the second surface (30) facing towards the measurement head (60), and the measurement beam (61) propagates through a medium (63) of the optical element (10).

4. The positions of the first, second and third target points (26, 27, 28, 36, 37, 38) on the first surface (20) and the second surface (30) facing the measuring head (60) are: the refractive index of the medium (63) of the optical element (10); the angle of incidence of the measurement beam (61) relative to the first, second, or third target point (26, 27, 28, 36, 37, 38); and 4. The method of claim 3, wherein the distance is determined based on a local surface profile of at least one of the first surface (20) and the second surface (30) in the region of the first, second or third measurement point (21, 22, 23, 31, 32, 33) and / or the target point (26, 27, 28, 36, 37, 38).

5. A first measurement path (25, 35) and a second measurement path (29, 39) are provided on the measurement surface (11).

5. The method according to claim 1, wherein at least two of the first, second and third measurement points (21, 22, 31, 32) are located on a first measurement path (25, 35) and at least one of the first, second and third measurement points (23, 33) is located on a second measurement path (29, 39).

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

7. 7. The method according to claim 5 or 6, wherein the first measurement path (25, 35) and the second measurement path (29, 39) are concentric with respect to the optical axis (c) or the reference axis (51) of the optical element (10).

8. 8. The method according to claim 1, wherein the optical element (10) is attached to a mount (40) arranged on a measurement stage (50), the measurement stage (50) being rotatable about an axis of rotation (53), and wherein at least one of the radial position (r) and orientation (a, b) of the mount (40) relative to the axis of rotation (53) is adjusted to minimize at least one of the eccentricity (D) and tilt (T) of the optical element (10).

9. 9. The method according to claim 1, 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 of the first surface (20) and the second surface (30) is determined.

10. Further comprising measuring the thickness of the optical element (10) along the optical axis (c), the thickness measurement comprising: Aligning a measurement head (60) along or parallel to the optical axis (c) of the optical element (10); directing a measurement beam (61) along or parallel to the optical axis (c); moving the focused measurement beam (61) along or parallel to the optical axis (c); and detecting a coincidence of a focal zone (68) of the focused measurement beam (61) with the first surface (20) and / or the second surface (30).

11. An interferometric measuring device (1) for measuring a surface (20, 30) or profile of an optical element (10), comprising: a light source (2) configured to emit a measurement beam (61); a mount (40) for fixing the optical element (10); a measurement head (60) connected to the light source (2), configured to direct a measurement beam (61) to a measurement surface (11) of the optical element (10) and further configured to receive a measurement beam portion (62) reflected from the measurement surface (11); a measurement head movable relative to the mount (40) to direct a measurement beam (61) to at least a first predetermined measurement point (21, 31), a second predetermined measurement point (22, 32), and a third predetermined measurement point (23, 33) on the measurement surface (11); a detector (4) connected to the measurement head (60), the detector configured to detect measurement beam portions (62) reflected at at least a first measurement point (21, 31), a second measurement point (22, 32), and a third measurement point (23, 33); a signal analyzer (5) 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 a third measurement point (23, 33), and a signal analyzer further configured to determine at least one of an eccentricity (D) and a tilt (T) of the measurement surface (11) relative to a reference axis (51) based on the position of the first position and the third position.

12. 12. The measuring device of claim 11, wherein the mount (40) is arranged on a rotating measuring stage (50) defining a reference axis (51), and at least one of the radial position (r) of the mount (40) and the orientation (a, b) of the mount (40) relative to the reference axis (51) is adjustable.

13. 13. The measuring device of claim 12, further comprising a controller (6) operable to adjust at least one of a radial position (r) of the mount (40) and an orientation (a, b) of the mount (40) relative to a reference axis (51) based on at least one of an eccentricity (D) and a tilt (T) of the measurement surface (11).

14. The optical element further includes a measurement head controller (66) operable to move and / or align the measurement head (60) relative to the mount (40), wherein the measurement head controller (66) is operable to move and / or align the measurement head (60) relative to the mount (40), and to measure the positions of at least a first measurement point (21, 31), a second measurement point (22, 32), and a third measurement point (23, 33) on the measurement surface (11) facing away from the measurement head (60), and the measurement head controller (66) is operable to move and / or align the measurement head (60) relative to the mount (40), and to measure the positions of at least a first target point (26, 36), a second target point (27, 37), and a third target point (28, 39) on the first surface (20) and the second surface (30) of the optical element (10) facing towards the measurement head (60) on the opposite side from the measurement surface (11). and a third target point (28, 38), each of the first, second and third target points (26, 27, 28, 36, 37, 38) correlating to one of the first, second and third measurement points (21, 22, 23, 31, 32, 33), such that a measurement beam (61) entering the medium (63) 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 internally retroreflected at the first measurement point (21, 31), the second measurement point (22, 32) and the third measurement point (23, 33), respectively.

15. A computer program comprising instructions which, 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 carry out the steps of the method according to any one of claims 1 to 10.

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