EP3784980A1

EP3784980A1 - Method and device for testing geometric properties of optical components

Info

Publication number: EP3784980A1
Application number: EP19734666.1A
Authority: EP
Inventors: Reik Krappig; Max Riediger; Niels König
Original assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Current assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Priority date: 2018-04-23
Filing date: 2019-04-16
Publication date: 2021-03-03
Also published as: WO2019206371A1; DE102018109649A1

Abstract

The invention relates to a method and a device for testing geometric properties of optical elements made of plastic or glass by means of interferometry, in which optical coherence tomography is used and a volume scan of the optical element to be tested is recorded. In a preferred embodiment, in order to determine the shape of a second functional surface (3) arranged behind a first functional surface (2) in the incidence direction of the measuring beam (4), the refraction at the first functional surface (2) is taken into consideration, wherein the respective normal vectors (N_n) or the local curvatures of the first functional surface (2) are determined at points of impingement (P_n) of the measuring beam (4) on the first functional surface (2).

Description

Method and device for testing geometric properties of optical components

The invention relates to a method and a device for testing geometric properties of optical components according to the preamble of claim 1 or according to the preamble of claim 11.

Optical systems, especially micro-optical systems, are part of many devices and can contribute significantly to safety, health and safety

Comfort in daily life as well as in technical applications contribute. Examples include the mobile phone camera, optical couplers for communication, endoscopes in medical technology and sensor systems in vehicles, e.g. Passenger cars. However, the optical function of such systems can only be ensured if the properties of the individual components meet the specifications. The optical properties of a component are essentially determined by the geometry and nature of the associated functional surfaces and by the refractive index also by the material used. In this respect, innovative optical components are characterized above all by improvements in the material properties or or and by novel geometries. Both options are off

metrological view usually associated with challenges that often lead to the specified component properties must be tested with great effort or under certain circumstances a test in the specified tolerance range is not possible.

In terms of component geometry, the decisive criteria are, on the one hand, the shape of the optical functional surfaces and, on the other, their position in the overall system. Both aspects are addressed by the invention.

With regard to the form testing of demanding optical functional surfaces, on the one hand there is the challenge that the optical components, which become increasingly complex as the development progresses, are characterized by ever greater edge steepnesses. The edge steepness describes the inclination of the surface compared to a plane perpendicular to the optical axis. Especially with micro-optical aspheres and freeform lenses, which are usually produced by plastic injection molding, In some cases considerable flank slopes of eg 20 ° to 60 ° can occur. Already flank gradients from 20 ° present large measuring systems

Flerausforderungen.

Tactile measuring systems for shape testing are known from the prior art, but their application is subject to restrictions in the interaction between probe and surface to be tested. The desired geometry information is detected by the relative movement of a stylus tip pulled over the component surface. In steep areas, it may happen that the probe touches the specimen surface not only with the decisive for the measurement front end but with its edge and thus the measurement is falsified. Although this error can often be detected directly in the measurement result, it limits the usability of tactile methods for shape testing to edge steepnesses of approx. ± 25 °.

There are also limits to the maximum detectable edge steepness for optical methods known from the prior art, since the radiation used for the measurement must be conducted back to the sensor. The combination of diameter and working distance of the objective, i. its numerical aperture, determines at which maximum inclination of the specimen surface of the reflected measuring beam can get back to the sensor or lost. Because of further practical restrictions with respect to working distance, reflectance, etc., even with high-quality lenses, edge steepnesses of only approximately ± 30 ° can be measured.

Due to the problems described in connection large flank slopes, a kinematics is provided in various variants of the method that allows a possible orthogonal alignment between Prüflingsoberfläche and the probe tip or another sensor or - in the case of optical methods - the emitted radiation. This approach has long been in conventional

Stylus instruments integrated or is used in systems that deliver optical sensors or lenses highly accurate and thus can capture even very steep geometries.

The known systems have a number of disadvantages. So tactile systems are already due to the possible mechanical impairment, eg by generation visible and undesirable furrows on the sample surface, problematic. When using often complex external kinematics, which include rotation, pivot and rotation axes and reference mirror, results for the

Measurement data acquisition a tolerance chain, which must be minimized with high equipment technical effort, so that form measurements with sufficiently low

Uncertainties are feasible. This usually results in that

required measuring times at least one minute and therefore for typical cycle times in the e.g. Injection-based production of optical components are unsuitable.

To different functional surfaces of an optical element, e.g. Both sides of a lens to make accessible to the sensors, in the prior art elaborate clamping systems are used in which the specimen is held and which can be turned and positioned with high precision. In contrast, with a corresponding equipment expense also a successive

provided sensor pair for the upper and lower surface and thus the measurement times are reduced.

In the following, known, optically working test methods for functional surfaces are briefly outlined:

Chromatic confocal point sensors use chromatic aberration, ie the dependence of the focal length on the wavelength of the radiation

Aberration of lenses minimized in conventional optical systems (e.g., DE10 2016 115 827 A1). Since radiation of lower wavelength (blue) has shorter focal lengths, than radiation of higher wavelength (red) can over a

connected spectrometer analyzes the reflected back from the specimen surface radiation and thus the sought height information can be measured. By means of a lateral positioning of the sensor above the specimen surface, it is possible to generate a flat test statement, whereby roughness measurements are not possible due to restrictions for the lateral resolution. Optically flat methods generate the check information

different way. Thus, in Formprüfinterferometrie the deviation of the specimen surface is examined to a reference wavefront, which was previously generated with a correspondingly highly accurate lens (for example, DE 10 2017 217 372 A1). If these lenses are spherical, they can be used to test all spherical objects as well as those aspheres that deviate only slightly from the spherical shape, depending on the diameter of the specimen. For strongly aspherical specimens or free-form surfaces, the individual reference wavefront must be determined by means of a computer-generated

Flologramms CGFI be generated.

Confocal microscopy (e.g., EP 0 646 768 A2) utilizes a specially designed beam path in which only the radiation which is at the exact focal point of the objective used is returned to the sensor. If the vertical position of the objective is precisely adjusted by means of piezo actuators, the flea information of the relevant specimen surface can be detected.

The high lateral resolution of optical surface measuring methods and the

Confocal microscopy allows a combined shape and roughness measurement

Subareas of the specimen surface. Although the lateral resolution scales

opposite to the selected measuring range, but also larger areas can be detected by the stitching of individual measurements. For this purpose, a certain overlap range of the individual measurements is used, which can be up to 20%. Stitching artifacts nevertheless still occur, in particular with feature-poor surfaces, that is to say calculation errors during the computational alignment of the individual measuring fields, which may unduly impair the measurement result.

As already mentioned, in addition to the form test, the examination of the orientation of optical functional surfaces to each other, for example of the two functional surfaces of a lens, another important aspect. Despite precise individual surfaces, the quality of the function of the lens is crucially dependent on how the two surfaces are oriented to one another. This applies both to the functional surfaces of a single optical element (eg front and back of an optical attachment) and to the individual elements of a multi-component system. Possible errors in the orientation of the Functional surfaces to each other can exist in decentring and tilting. During decentration, the optical axis of the functional surface is laterally offset parallel to the axis of the system, whereas it is twisted by any point during tilting. These component errors are directly related to the

Production process and may be e.g. on the parallelism of the

Forming tools or the straightness of their leadership are attributed.

Information about the orientation of functional surfaces to each other according to the prior art can be detected by measuring only with considerable effort, e.g. by checking the orientation of the respective optical axis by means of a

Autocollimation telescope (in the case of spherical surfaces) and, if necessary, with additional sensors for detecting the impact (for aspherical surfaces). In some cases (for example free-form surfaces), the orientation can not be detected at all. It is always an iterative process in which the individual functional areas are recorded successively.

The detection of such errors can be done both by checking the geometry and by checking the function.

According to the state of the art, the deviation of the center of curvature in the geometry test, for example, via an autocollimator

rotationally symmetric surface to the optical axis of the system detected, so as to be able to measure any incorrect positioning of this surface can (for example, DE 10 2006 052 047 A1). Even in multicomponent systems, so can the individual

Functional areas and thus the overall system are characterized successively.

For functional testing, wavefront sensors are frequently used, which use a microlens array to image the local slopes of the wavefront and thus the light-shaping properties of the optical system on a CCD camera (eg DE 103 48 509 A1). The thus reconstructed wavefront can serve to provide more detailed information about the system properties. For example, the measured wavefront can be decomposed into its orthogonal polynomials, the so-called Zernike coefficients. With a manageable number of such coefficients The measured wavefront can be simulated in a good approximation. Of the

However, the essential advantage of this coefficient representation lies in the fact that certain coefficients permit conclusions to be drawn about certain system states.

For example, the coma term of the wavefront correlates directly with the

Decentralization of the system components. These relationships are z. T. linear and can be used to correct the positioning of the

Calculate and implement components to each other.

Optical Coherence Tomography (OCT) is a well-known variant of white-light interferometry that employs short-coherent light. The short coherence length of the light is required for high axial resolution. One differentiates between different OCT procedures. First, the Time Domain OCT (TD-OCT) is known, in which the optical path in the reference arm of the interferometer in its length must be changed. A disadvantage is the required movement of mechanical parts, which costs time and brings with it the risk of collisions or vibrations. Without mechanical movement, the Frequency Domain OCT (FD-OCT) comes out, for which preferably two alternative methods are used. SD-OCT (Spectral Domain Optical Coherence Tomography) uses a broadband radiation source and a spectrometer at the detector. Alternatively, SD-OCT (Swept Source Optical Coherence

Tomography) are used, in which the radiation source is tunable. The detector then uses a balanced photodetector (balanced photodetector). In both alternatives, the signal received is a

Fourier transformation performed.

OCT has hitherto been used essentially in the medical field, e.g. for measuring soft tissue volume or in the field of ophthalmic diagnosis and treatment. In general, OCT distinguishes between A-Scan, B-Scan, and C-Scan. In the following, an A-scan is understood to mean a measurement in the depth, in which no scanning movements of the measuring beam in lateral

Direction to be carried out. In the case of the B-scan, a lateral scanning movement of the measuring beam takes place along a straight line in such a way that a sequence of A-scans results in a section through the examined sample. A C-scan is a sequence of several adjacent B-scans to produce a volume scan of the object under investigation or a portion of the object.

From DE 10 2009 006 306 A1 a device for calibrating or evaluating the performance of a refraction platform is known which has a model eye and at least one reference element. The model eye is formed by a front cap and a hemisphere covered by the cap, which cap is nearly transparent to light used when using the device. The cap can e.g. be realized by a film, a vapor-deposited layer, injection molding or a contact lens. The hemisphere is spaced from the cylindrical body by the variable thickness of a slot. The slot may be used to insert further elements, e.g. an iris model, serve. The cap, hemisphere and body are each made of different materials with different refractive indices. The outer surface of the cap and the interface between the cap and hemisphere reflect incident light which enables measurement of the material thickness of the cap, e.g. by OCT measurement. The thickness can be measured during a shape changing process or before and after. It is not revealed by means of OCT in addition to the

Thickness measurement also to obtain information about the shape and orientation of the interfaces involved.

From DE 103 92 656 B4 is an interferometer system for determining a

Three-dimensional workpiece surface known. The white light working

Interferometer is moved with a three-axis structure over the workpiece, wherein the reflected upon impact of a focal point of the workpiece surface radiation is evaluated. In order to make a statement about the direction of movement of the measuring head relative to the workpiece, is in various

Embodiments of the use of a plurality of spaced focal points of radiation of different frequencies proposed. The document refers to the time domain (time domain) method used as optical coherence tomography. It is only the information about a

determined three-dimensional surface. The measurement method does not provide volume information about the measured object, which obviously is not

is (partially) transparent. EP 1 744 119 A1 discloses a method for analyzing the shape, dimension or topography of an object, the FD-OCT being used, using a tunable radiation source (SS-OCT). In one

Embodiment illustrates the application of the tomography method for measuring surfaces in the interior of a depression. It is also disclosed that with this method it should be possible to examine several partially reflecting surfaces of a partially transparent object, without the relevant ones

To illustrate the approach in more detail.

From DE 10 2010 032 138 A1 an OCT-based, ophthalmological

Measuring system with a scanner unit known, which is a SS-OCT system (Swept Source OCT), wherein the dimming time dA / dt of the light source to the desired maximum measurement depth and the frequency of the axial modulation of the scanner unit is adjusted. This is intended to compensate or at least minimize the influence of occurring axial modulations of the scanner device. The measuring system is used to determine distances and to depict eye structures

used. It is disclosed that the system could be used in fields other than the field of ophthalmology, without, however, giving a concrete example.

The present invention is based on the technical problem of providing a method and a device for testing optical elements, which represent an alternative to the prior art and in particular the

Possibility to allow complete characterization of an optical element in a single measurement.

This object is achieved in a method of the type mentioned by the

Characteristics of claim 1 in a device solved by the features of claim 11. Advantageous embodiments will be apparent from the dependent

Claims. Thus, it is proposed that optical coherence tomography be used to test geometric properties of plastic or glass optical elements, whereby a volume scan (C-scan) of the optical element to be tested is recorded.

The optical coherence tomography, hereinafter referred to as OCT, is very flexible with respect to the geometry of the optical element to be tested. By the high

Sensitivity of the OCT, a small part of the specimen of backscattered light is sufficient to obtain a usable measurement signal. As a result, specimens with a high edge steepness can be measured in particular, which can be realized only with great effort with the above-described alternative methods according to the prior art. Furthermore, the process is fast and can characterize several functional surfaces with only one measurement within a few seconds. Moreover, in the method according to the invention, a high-precision alignment of the test object with respect to the measuring system is eliminated. Another advantage of the method is the relatively low technical complexity for realizing an OCT system. The measurement takes place in backscatter, so that the integration can be done comparatively space-saving, which is especially for

Machine-integrated or automated applications offers advantages. The implementation of optical paths with the help of optical fibers allows an im

Compared to the prior art robust and at the same time flexible and compact design with the smallest possible number of moving parts.

The method and the device according to the invention can e.g. advantageous for optical testing in the field of optical or micro-optical

Components, e.g. Aspheres or free-form lenses, are used, in particular for attachment optics, e.g. with LED lighting optics. Especially in the replicative production, there is a need for a quick inspection method, which can be used for a variety of geometries without special adaptation

Delivers results. Due to the relatively compact construction of OCT measuring devices and the possible use of fiber optics, the method according to the invention and the device according to the invention can be easily incorporated into existing production processes, for example by integration in injection molding machines. The method according to the invention can be carried out such that the shape of at least two functional surfaces of at least one optical element is tested. With the detection of the shape of multiple functional surfaces in only one measurement, the orientation of the at least two functional surfaces relative to one another is automatically determined.

The combined detection of both the shape and the orientation of the

Functional surfaces to each other allows a complete metrological

Characterization of optical elements in geometrical terms in only one measurement. Due to the characteristics of the tomographic acquisition of both functional surfaces, the new method thus makes it possible to detect not only enormous edge steepnesses but at the same time also to characterize the orientation of the functional surfaces.

When testing at least two functional surfaces they may belong to the same optical element, preferably an optical lens. The functional surfaces to be tested can also belong to different optical elements.

When testing two in the direction of irradiation arranged one behind the other

Functional surfaces may be the image of the rear functional surface due to the

Refractive properties of the material of the optical element at the front

Functional area are distorted. For known refractive properties of the optical element, this may preferably be taken into account by means of a software-based algorithm. The same applies to the investigation of three or more successively arranged functional surfaces, namely by taking into account the refraction at each transition between materials of different

Refractive indices.

The consideration of the refraction properties for the determination of the shape of the back functional area can be done separately for each A-scan of the volume scan (C-scan) considering adjacent A-scans by means of ray tracing

(Raytracing). For this purpose, first the normal vector on the Measuring system facing functional surface to be determined at the point of impact of the measuring beam. This can be done for example by triangulation or local surface adaptation. Subsequently, with the incident vector of the measuring beam, the

apparent distance between front and rear functional surface in the A-scan and the refractive indices of the materials surrounding the front functional surface using Snell's law of refraction, the geometric location of the rear functional surface in the coordinate system of the measured data determined.

Alternatively, a full surface fit may be made to each of the front and back functional surfaces, i. to the complete or substantially complete functional area, e.g. adapted by using a mathematical model such as the aspheric equation or using Zernike polynomials, an area. Subsequently, the determination of inclination values or normal vectors on a variable number of support points can be carried out in order to be able to correct the distortion of the rear functional surface in accordance with the above-mentioned ray-tracing method.

The measured data of the functional surfaces determined in the manner described may be e.g. directly compared with the data of an object model, e.g.

Determine deviations of the specimen of nominal data, or it can

characteristic test parameters are determined. In the case of a lens such test parameters may e.g. the center thickness, radii of curvature, aspheric coefficients, wedge angle or decentering.

In particular, the method according to the invention can be carried out so that the OCT is carried out in the frequency domain, i. FD-OCT (Frequency Domain Optical Coherence Tomography) is used. Thus, a change in the length of the reference arm of the interferometer can be avoided. In addition, the measurement can be performed faster compared to a TD-OCT. As a variant of FD-OCT, e.g. the SS-OCT or the SD-OCT are used.

In the following, a preferred embodiment of the invention

Method illustrated by figures. It shows

1 shows a known from the prior art measurement setup with an SD-OCT,

FIG. 2: a representation for clarification of a triangulation method for

Determination of a normal vector N,

3: the beam path of a refracted at a first functional surface

Measuring beam and

4 shows an illustration for explaining the correction of the determined course of a second functional area.

Fig. 1 shows a possible measuring device with an SD-OCT. With the exemplary method, two functional surfaces 2 and 3 of an optical element, namely a lens 1, are to be determined by means of a C-scan with respect to their shape and orientation. The radiation for generating a measuring beam 4 comes from a

broadband radiation source 5 for short coherent radiation, e.g. one

Superluminescent diode, and is distributed via optical fiber 6 and a fiber coupler 7 in the measuring device. The radiation passes as a reference path radiation 8 a reference path 9 with a reference mirror 10 and as a measuring beam 4 an object path 11 with scanner mirrors 12 and a scanner lens 13. By means of the scanner mirror 12, the measuring beam 4 for performing a scanning movement in two lateral directions be guided over the lens 1. The backscattered from the functional surfaces 2 and 3 radiation of the measuring beam 4 is combined in the fiber coupler 7 with the reference path radiation 8, so that it comes in a spectrometer 14 for interference. In the spectrometer 14, the radiation is spectrally decomposed at an optical grating 15 and directed to a line scan camera 16.

The measuring device described and the beam guidance are generally known from the prior art for SD-OCT method. For the inventive method, alternative OCT methods can also be used. As shown, the measuring beam 4 of the SD-OCT structure is directed onto the lens 1 to be examined in such a way that the two functional surfaces of the lens 1, hereinafter referred to as first functional surface 2 and second functional surface 3, are arranged one behind the other in the direction of radiation Function surface 1 is designated the one on which the measuring beam 4 impinges first.

A C-scan is performed. The resulting tomographic image can be used to contrast enhancement and noise filtering in general for others

Be re-processed applications known methods, e.g. using Gaussian filter or histogram spread. Subsequently, it is determined from which locations of the examined lens radiation was scattered back, which has led to local intensity maxima of the interferences generated in the measuring device. The distribution of the intensity maxima now contains information about the functional surfaces 2 and 3 of the lens 1. Ggf. In order to improve the precision, the axial position of the local maxima can be determined with subpixel accuracy, e.g. about a Gauss fit. From the

Intensity maxima can be determined for both functional surfaces 2 and 3 forms, which can be stored and further processed, for example, as a point cloud.

However, the point cloud of the second functional surface 3 must be corrected, since the radiation component scattered back by the second functional surface 3 is refracted at the first functional surface 2. To correct the data of the second functional area 3, ray tracing is used. For this, first the

Normal vector N on the first functional surface 2 at the impact points P of the

Measuring beam 4 can be determined. This can be done for example by triangulation or local surface adaptation. During triangulation, which is shown in Fig. 2 is a schematic for a small number of impact points P ₀ to P ₈ in the first functional surface 2, the point cloud triangles are clamped between the identified points Pi to P _8. Normal vectors D ₀ to D _{7 of} the triangles arranged around the mean point of impingement P ₀ are calculated, and then, by means of interpolation, the normal vector N ₀ at the central one

Impact point P ₀ to determine. The interpolation can take place via a weighted averaging of the surrounding normal vectors D ₀ to D ₇ . For this are from the state of Technique Various approaches in connection with the curvature estimation of triangulated point clouds known (see, for example, Günter Grosche, Viktor Ziegler,

Eberhard Zeidler, Dorothea Ziegler: "Teubner - Paperback of Mathematics, Part 2",

8th edition, Springer-Verlag (2013), p. 103). In this way, become everyone

Impact points P _{n of} the point cloud determines the normal vectors N _n . This results in curvatures and thus a surface course for the first functional surface 2.

As an alternative to the described triangulation method, an adaptation of mathematical surfaces to the point cloud can be carried out in the local environment of the impact points in order subsequently to determine the normal vector at the point of impact from the adapted surface.

The well-known course of the functional area 2 is now used to under

Taking into account the given refraction of the measuring beam 4 to correct the thus determined distorted form of the second functional surface. With the help of

Snell's law of refraction, using known quantities, namely the direction of the incident measuring beam 4, in FIG. 3, is provided with the vector symbol /, the refractive indices n j and n _t of the first functional surface 2

enclosing materials, namely in the example of the lens 1 air and glass or plastic, and the previously calculated normal vector JV of the vector T of the propagation of the measuring beam after piercing the first functional surface 2 determined (see Fig. 4). The formula suitable for this is:

Fig. 4 illustrates how the _n thus determined for each impact point P vector T _n and the optical path length l _n between the point of incidence P _n of the measuring beam 4 on the first functional surface 2 and the apparent impact point Q _n at the second Functional surface 3 using the following calculation, the actual, that is undistorted geometric position Q ' _{n of} the point of impact of the measuring beam 4 on the second functional surface 3 can be determined.

In this way, by reconstructing the radiation patterns at all points P _n, the actual shape and position of the second functional surface 3 represented by the set of points Q ' _n can be calculated. For the

Reconstruction of the radiation profiles is not detected by the functional surfaces reflected light but the reflected scattered light, since

Due to the measuring principle, only the light beams contribute to the OCT signal, which penetrate the same way into the scanner lens 13 of the OCT system on which they have leaked. A tracking of the measuring beam 4 using the

Reflection law is therefore not made, since measuring principle, only the light rays contribute to the OCT signal, which penetrate in the same way in the scanner lens 13 of the OCT system on which they have leaked.

For the desired information about the shape of the rear functional surface 3, this does not have to be triangulated. After the distortion correction, the points Q ' _n, for example, can be directly compared with target data or it is adapted to the points Q' _n a suitable model (eg Asphere equation or Zernike polynomials) from which then characteristic characteristics of the lens surface can be derived (For example, radius of curvature, aspheric coefficients, possibly form error).

Since both functional surfaces 2 and 3 were determined in their shape with the measurement carried out, their relative orientation to each other is also known. LIST OF REFERENCE NUMBERS

1 lens

2 first functional area

3 second functional area

4 measuring beam

5 radiation source

6 light guides

7 fiber couplers

8 reference path radiation

9 Reference path

10 reference levels

11 object path

12 scanner mirrors

13 scanner lens

14 spectrometers

15 optical grating

16 line scan camera

P _n impact points of the measuring beam on the first functional surface

D _n normal vectors triangles

N _n normal vector impact point P _n

Q _n impact points of the measuring beam on the second functional surface (distorted)

Q ' _n impingement points of the measuring beam on second functional surface (undistorted)

Claims

Method and device for testing geometric properties of optical components Claims

1. Method for testing geometric properties of optical elements made of plastic or glass by means of interferometry,

used in the optical coherence tomography and a volume scan of the optical element to be tested is recorded.

2. The method according to claim 1, characterized in that the respective shape of at least two functional surfaces (2, 3) is checked at least one optical element.

3. The method according to claim 2, characterized in that the alignment of the at least two functional surfaces (2) is determined relative to each other.

4. The method according to claim 2 or 3, characterized in that the

Functional surfaces preferably to the same optical element, preferably an optical lens (1) belong.

5. The method according to any one of claims 2 to 4, characterized in that for determining the respective shape of at least one of the functional surfaces (2, 3) in the direction of irradiation of a measuring beam (4) seen before this functional surface (2, 3) given refraction of the radiation is taken into account.

6. The method according to claim 5, characterized in that ray tracing is used.

7. The method according to claim 6, characterized in that for determining the shape of a beam in the direction of the measuring beam (4) behind a first functional surface (2) arranged second functional surface (3), the radiation refraction at the first functional surface (2) is taken into account Impact points (P _n ) of the measuring beam (4) on the first functional surface (2) the respective normal vectors (N _n ) or the local curvatures of the first functional surface (2) are determined.

8. The method according to claim 7, characterized in that the

Normal vectors (N _n ) are determined by means of a triangulation method.

9. The method according to claim 7, characterized in that the local

Curvatures of the first functional surface (2) is determined by means of surface adaptation.

10. The method according to any one of the preceding claims, characterized in that FD-OCT (Frequency Domain Optical Coherence Tomography), preferably SS-OCT (Swept Source Optical Coherence Tomography) or SD-OCT (Spectral Domain Optical Coherence Tomography), is used.

11. An apparatus for testing geometric properties of optical elements made of plastic or glass by means of an interferometrically operating measuring device, wherein the measuring device is adapted to perform a method according to one of claims 1 to 10.