Patent Description:
In many fields of use, there is a need to gauge surfaces of objects and hence also the objects themselves with high accuracy. This applies in particular to the manufacturing industry, for which the gauging and checking of surfaces of workpieces is very important.

For these applications, there is a number of existing measuring devices which are designed for specific tasks and are also designated as coordinate measuring devices or machines. These measuring devices gauge the surface by producing mechanical contact and probing the surface. Examples of this are gantry measuring machines, as described, for example, in <CIT> or <CIT>. Another system is based on the use of an articulated arm whose measuring sensor arranged at the end of the multipart arm can be moved along the surface. Articulated arms of the generic type are described, for example, in <CIT> or <CIT>.

Approaches for non-contact gauging have already been pursued in the prior art. One approach utilizes white light interferometry for high-precision gauging. Here, the application either employs scanning, i.e. by displacement of the interferometer, and therefore takes place slowly or, in the case of spectrally resolved detection, as a rule with limitation to a measuring range of a few mm.

<CIT> discloses a system for gauging surfaces using optical coherence tomography and a frequency-modulated source. Here, a fibre ring laser is made tuneable by an acoustically tuneable filter element. The laser radiation is then used for interferometric gauging of surfaces in a common path interferometer, i.e. an interferometer which uses at least partly the same components or beam paths for measuring radiation and reference radiation. The reference distance here is provided by a reflection in the measuring arm of the interferometer. A calibration interferometer is used for calibrating the wavelength.

There are different problems associated with such interferometric measurement methods and devices. An important source of error is the occurrence of so called speckles. The speckle effect originates from the coherent superposition of light with different relative phases reflected from the surface within the resolution cell of the scanning device. Such phase differences occur for example from rough surfaces where the height variations are on the scale of the used wavelength and the lateral size is smaller than the resolution cell. Due to the disturbing speckle effect, the measured profile shows outliers which are not present in the real surface profile. Known methods and devices for interferometric measurement of surfaces are not able to adequately deal with these errors. <CIT> discloses a method, wherein the distances are determined from the portions of the spectra, which are above a certain amplitude threshold, while the rest of the spectra is not considered for the evaluation. This method reduces the outliers; however, it cannot suppress them completely as a reduced dataset is inherently less accurate.

<CIT> discloses a frequency modulated continuous-wave laser radar for distance measurements wherein deviations of the interference signal amplitude are determined and distance measurements are disregarded based on the deviations.

Moreover since storing the raw spectra would increase the required data volume by orders of magnitude this information is typically discarded at some stage. Thus, in a later processing step it is hard, if not impossible to determine, whether a part of profile is influenced by a speckle artefact treated such a way.

An object is to provide an improved measuring method or measuring device for gauging surfaces or for determining surface topographies.

A further object is to provide a measuring method or measuring device which adequately deals with, in particular speckle induced, measurement disturbances.

These objects are achieved by the subjects of the independent claims or of the dependent claims, or the solutions are further developed.

The invention pertains to a method for, in particular industrial, measurement of a surface, prefarably according to the principle of Optical Coherence Tomography, whereby based on interferograms, e.g. by analyzing the respective modulation frequency, distances to points of the surface are measured. The method comprises generating a laser beam, emitting the laser beam onto the surface, whereby the laser beam is focused on a point of the surface, receiving at least a part of the laser beam, reflected by a respective point of the surface, and generating an interferogram by superposition of the received laser beam with a reference laser beam.

The method further comprises the step of classifying of measurements as valid or non-valid based on evaluation of amplitude change of a respective interferogram. Said otherwise, a respective interferogram or the distance derived therefrom are classified as valid or non-valid based on a test of amplitude change of the respective interferogram. Preferably, the classification serves for sorting out or tagging/marking of measurements disturbed by occurrence of laser light speckles.

Preferably, measurements classified as non-valid are tagged and stored as non-valid or deleted in real-time during measurement. Optionally, a respective interferogram is tagged or deleted before any processing for distance calculation, i.e. non-valid measurements are recognized in due time before any further processing is effected which would be "waste" of processing power as the result is too flawy anyway. As another option, the method comprises generating a profile of the measured surface wherein non-valid measurements/distances are graphically marked. Alternatively or additionally, non-valid distances are excluded from the profile, wherein preferably continuity of the profile is maintained by interpolating between non-excluded measurements.

According to the invention, a respective measurement is classified as non-valid if the result of the evaluation is above one or more defined thresholds, wherein preferably the threshold is defined in a calibration procedure with measuring of one or more standard surfaces. For example, if the phase change of a respective interferogram exceeds a defined threshold, the respective interferogram is categorized as non-valid.

As another option, the evaluation comprises searching for a disturbance of phase and/or amplitude of a respective interferogram. Alternatively or additionally, the evaluation comprises determining a degree of fluctuation of the phase and/or amplitude of a respective interferogram. Said otherwise, it is evaluated how much phase and/or amplitude variation or drift is present in a respective interferogram. As another option, the evaluation comprises comparing a phase and/or amplitude chart of a respective interferogram with an ideal phase and/or amplitude chart. The charts can be embodied as graphs, functions, tables etc. as form of data embodiment for derivation of a deviation of the measured phase and/or amplitude from ideal values.

In a preferred embodiment, the evaluation comprises calculating the unwrapped phase of a respective interferogram, fitting a linear function through the interferogram phase (chart), subtracting the linear function from the phase, calculating the Standard Deviation and classifying the interferogram based on the Standard Deviation. If for example the Standard Deviation exceeds a predefined threshold, the respective interferogram or measured distance is classified as non-valid.

According to the invention, the evaluation comprises detecting if the amplitude of a respective interferogram is temporarily below an amplitude threshold. The amplitude threshold can be an absolute threshold. Alternatively, the threshold is a relative one, depending on a maximum amplitude of a respective interferogram. According to the invention, a respective measurement is classified as non-valid if an interferogram fraction with amplitude below the amplitude threshold is above a fraction threshold. Said otherwise, according to the invention, it is not only evaluated if there is amplitude below the amplitude threshold is present in the interferogram, but it is also evaluated to what extent such low amplitude is present. If for example, the low amplitude is detected but it lasts not longer than a predefined period, the measurement is classified "valid". Optionally, an amplitude based weighting factor for phase information of a respective interferogram classified as valid is applied for calculating the distance to the point. As a further option, the weighting factor is directly dependent on the amplitude of a respective interferogram and/or is set as zero if the amplitude is below an amplitude threshold. The amplitude threshold for phase weighting may be (but has not to be) identical to the above mentioned amplitude threshold for classification.

The invention also pertains to an interferometric measuring device designed for measuring a surface, in particular according to the principle of Optical Coherence Tomography, the device comprising a laser for generation of a laser beam, a drive for guiding a laser beam emitting measurement head above the surface such that the laser beam is scanning the surface point-by-point, a receiver for receiving at least part of the laser beam reflected by a respective point of the surface and an interferometer for generating an interferogram by superposition of the received laser beam with a reference laser beam as well as a signal processor for measuring a distance to a respective point based on a respective interferogram.

According to the invention, the signal processor, e.g. a Field Programmable Gate Array (FPGA), is configured to classify measurements as valid or non-valid based on an evaluation of phase change and/or amplitude change of a respective interferogram.

Additionally, the invention also pertains to a non-transitory computer program product, comprising program code which is stored on a machine-readable medium, in particular of an interferometric measuring device according to the invention, and having computer-executable instructions which when executed cause a computer to perform the measurement method according to the invention.

The present invention allows advantageously to identify and sort out interferograms resp. distances derived therefrom which have (too much) errors, in particular errors resulting from speckles. Thus, invalid measurements are dismissed from the beginning and do not have to be erased afterwards. A resulting measured surface profile does not show any (speckle induced) outliers like resulting from methods/devices of the prior art resp. such outliers can already be marked as flawy measurements during creation of the profile.

A method according to the invention and a measuring device according to the invention for gauging surfaces are described or illustrated in more detail below, purely by way of example, with reference to working examples shown schematically in the drawing. Specifically,.

<FIG> illustrate a first example of an interferometric measuring device <NUM> and an according method for measuring of a surface <NUM>, whereby in the example, the measurement is based on the principle of Swept Source Optical Coherence Tomography (SS-OCT). Although the herewith in more detail described device and method are based on swept-source OCT the following aspects are applicable also for Fourier-domain OCT consisting of a white light source in combination with a spectrometer and line sensor on the detection side. The device <NUM> is designed for usage in the intended field of industrial coordinate measuring devices with free-beam measurements of a few cm using compact probe heads having diameters in the region of ruby spheres. In this frequency-modulated interferometry, a source which should as far as possible permit broadband tuning in a short time is used. Moreover, narrow-band characteristics with a coherence length of a few cm are required. The tuning of the source is e.g. referenced via a calibration interferometer whose length is known very precisely.

In <FIG>, a frequency modulated laser (swept source) <NUM> generates a laser beam <NUM> which is guided by optical fibers <NUM> to a measurement head <NUM> and emitted therefrom at the surface <NUM> to be measured. The laser source <NUM> is for example in the form of a fibre ring laser having an optical semiconductor amplifier as an amplifying medium and a tuneable filter element. If higher repetition rates are desired in the measurement, the fibre ring laser can be extended by a fibre length of several kilometres, the repetition rate corresponding to the inverse of the transit time of the light in the fibre ring. As a further possibility for setting up the laser source <NUM>, it is also possible to use an external cavity having a dispersive element, for example a grating or prism in combination with a moveable optical surface, e.g. a polygonal mirror, for fast tuning of the laser wavelength for the laser resonator. The tuneable element may be formed, for example, as a Fabry-Perot filter or as an acoustically tuneable fibre Bragg grating. Further elements are Semiconductor Lasers, tunable VCSELs (Vertical cavity surface emitting laser), distributed feedback lasers (DFBs), optical couplers or insulators, the use and integration of which in such a system are known to the person skilled in the art.

The laser beam <NUM> is focused at a spot or point on the surface <NUM>. The back reflected light is superimposed with light from a fixed reflector which serves as a local oscillator (e.g. the last optical surface of the measurement head <NUM>). Due to the modulation, light with different frequencies interferes depending on the distance D to the object <NUM> and the associated time delay. This results in a temporal amplitude modulation or interferogram whose frequency (beat frequency) is directly related to the distance D to a respective point of the surface <NUM>.

The resulting temporal amplitude modulation or interferogram is detected by a photo detector <NUM> and converted to a digital signal resp. digitalized interferogram <NUM> (see also <FIG>). Such an interferogram can be (completely) continuous. Alternatively, an interferogram comprises a couple of discrete segments, for example in devices with an DFB-array as swept light source. Each interferogram segment or "sub"-interferogram is assigned to the tuning-region of a particular DFB. In any case, for subsequent evaluation (classification of interferograms/measurements) as described below, a respective interferogram is considered as a whole, wherefore for example in case of discrete segments, the segments are stitched together before evaluation. Said otherwise, the signals e.g. from different tuning-regions are first combined.

A signal processing unit <NUM>, e.g. a FPGA or some other form of computer processor, provides the calculation of the distance D to the target surface <NUM> (more precisely: the targeted surface point) by analyzing the modulation frequency of the respective digitalized interferogram. In other words, the calculation of the distance D is primarily based on the phase information of a respective interferogram <NUM>.

By moving the laser spot over the surface <NUM> i.e. scanning (arrow <NUM>), a plurality of surface points and thus the height variation or profile of the surface <NUM> is measured. However, the measurement can be disturbed, i.e. the measured distance deviates from the real distance D, which will be explained in more detail with respect to <FIG>.

Referring to <FIG>, a measurement disturbance caused by speckles, i.e. position-dependent intensity variations, in coherent observation, is described. In the upper part of <FIG>, it is shown that the spot of the laser beam <NUM> on the surface <NUM> has always a certain size which corresponds to the resolution of the sensor defined by the PSF (point-spread-function) of the lens. In case of a rough surface <NUM>, height variations can occur within the resolution cell. The light 23r reflected from different parts of the illuminated area is sent to the detector <NUM> with different phase (in the example two phases P1 and P2 are illustrated). Depending on their relative phase the light can interfere constructively or destructively. Furthermore, the phase of the superimposed light 23r can differ significantly from the phase of the single parts. Said otherwise, this so called speckles originate from the coherent superposition of light 23r with different relative phase P1, P2 reflected from the surface <NUM> within the resolution cell of the scanning device. Such phase differences occur for example from rough surfaces <NUM> where the height variations are on the scale of the used wavelength and the lateral size is smaller than the resolution cell.

The resulting interferogram <NUM> (lower part of <FIG>), outputted by the detector <NUM> is disturbed (area graphically marked by circle <NUM>) and accordingly, the signal processor <NUM> calculates (or would calculate) a wrong distance. With the presented invention, such disturbed measurements are recognized which is exemplified with respect to <FIG>.

<FIG> shows an exemplary procedure for classification <NUM> of measurements as valid or non-valid. First, after generation of the interferogram <NUM>, the phase of the interferogram is calculated (step <NUM>). Thereafter, phase change is determined (step <NUM>). The phase change <NUM> serves as indicator of disturbance. If the phase change <NUM> is above a certain threshold (tested in evaluation step <NUM>), the interferogram resp. the measurement (signal) is classified as non-valid (6a), i.e. (too much) disturbed. Otherwise, the measurement is classified as valid (6b). Such a procedure is further exemplified with respect to <FIG>.

<FIG> shows in the upper part a chart of calculated (unwrapped) phase Φ (displayed over time) of a measured interferogram <NUM>. Ideally, without any disturbance, the phase line <NUM> would be a straight line. In case of a speckle disturbance the phase <NUM> deviates from the ideally linear function (marked by circle <NUM>). The phase change <NUM> is determined by fitting a linear function <NUM> through the interferogram phase <NUM> resp. phase line <NUM>, subtract the linear function <NUM> from the phase <NUM> and calculate the Standard Deviation of the phase Φ, this is a measure for the deviation (indicated by arrow <NUM>) of the phase <NUM> from a linear function <NUM>. By calculation of the Standard Deviation, in this example the phase change is determined, i.e. the Standard Deviation serves as a measure for phase change. Instead of using functional representations of the phase as shown, the comparison of the measured chart with an ideal chart is based on tables.

If the Standard Deviation <NUM> is larger than an adjustable threshold, then the interferogram <NUM>, and also the resulting distance value, are classified or recognized as "invalid". The value of the threshold can e.g. be determined by scanning tests on a typical rough surface <NUM>. Measurements classified as non-valid are tagged as non-valid or dismissed (deleted), preferably in real-time during measurement, i.e. either stored with a marker or completely removed, which is exemplified in more detail with respect to <FIG>.

<FIG> shows an exemplary final result of the measurement classification. The measured distances D or the measured profile <NUM> are shown, (grey or bright line). Due to the disturbing speckle effect, the measured profile <NUM> shows outliers <NUM> which are not present in the real distances resp. the real surface profile <NUM>' (black or dark line). Such outliers <NUM> have a typical shape similar to a pole in mathematical functions.

As these disturbances have been recognized by the previous evaluation <NUM> resp. the according measurements have been classified as non-valid, the distance values which are declared as "invalid" can be marked in the profile plot, indicated in the figure by dots <NUM>. If for example roughness parameters like Ra or Rz shall be determined from the profile <NUM>, those invalid points can be excluded from the calculation. Another possibility is to interpolate the profile <NUM> between adjacent "valid" distance points in order to obtain (or maintain) a continuous profile <NUM> without speckle disturbances, which is in <FIG> indicated by the thick straight line <NUM>. In embodiments of the method wherein non-valid measurements or distances are not marked in the surface plot, it is optionally waived to process the underlying interferograms <NUM> at all after they are classified as non-valid. Hence, they are not subject to distance calculation at all and optionally even completely removed by the processing unit, which can spare processing time and power and in case of complete and quick deletion save storage.

In either way, advantageously, the disturbances, primarily those by speckles, do not effect the final measurement result <NUM>. Hence, the presented method provides analyses of phase change as a quality marker to find and tag distance values of disturbed interferograms.

<FIG> shows an additional optional step. The figure is based on <FIG> and shows additional measurement points 11a "before" and "after" the previous invalid measurement points <NUM> are considered "invalid" though they have not been classified as "invalid" in the previous steps. In difference to the previous invalid points 11a, these additional invalid points 11a are considered as such because of their neighborhood to the initial invalid points <NUM>. Said otherwise, the invalid region of the original points <NUM> is "artificially" broadened with the additional points 11a. This optional step serves for example to guarantee that a speckle underlying the disturbance is completely considered, without any boundary speckle zone omitted.

As criterion, for example each <NUM>, <NUM> or so measurements "before" and "after" the original "invalid" measurements is declared "invalid", too, or a margin of the original invalid region is declared "invalid, e.g. <NUM>% or <NUM>% of the invalid profile at one or each end.

In accordance, a larger interpolation zone 8a than in the previous <FIG> is optionally established, comprising both the zone of values <NUM> initially classified as "invalid" as well as the zones of values 11a considered "invalid" due to their proximity to the initial values <NUM>.

<FIG> shows another example of a method for classification <NUM> of interferograms <NUM>. In a step <NUM>', the amplitude of the interferogram <NUM> is calculated. Next, in step <NUM>', any change of amplitude is determined. Amplitude fluctuation serves as indicator for validness. If the change or variation is above a certain threshold, tested in step <NUM>, then the interferogram <NUM> is classified as non-valid (step 6a). Said otherwise, if a too high fluctuation of amplitude is detected, e.g. a too strong drop of amplitude, then the respective measurement is declared "invalid". If on the other hand there is no drift above the threshold, then the interferogram <NUM> is regarded as valid (step 6b).

A test-threshold can be completely predefined, i.e. a value is predetermined as such. Alternatively, the test-threshold is semi-predefined in depending on a measured value of the interferogram, e.g. the maximum amplitude present in the interferogram or the general degree of phase change (gradient of line <NUM> in <FIG>). For example, the threshold is semi-predefined as a change which amounts to <NUM>% or <NUM>% of the maximum amplitude.

<FIG> shows a further exemplified illustration of an amplitude based classification. Shown is the amplitude A of an interferogram to be classified. The amplitude A is not unvarying but there is significant change (here in the middle, indicated by region B). Such a temporally amplitude drop down is e.g. caused by speckles. The interferogram shows that there is amplitude as low that it falls below a defined amplitude threshold <NUM> which serves as an "inverse" amplitude change threshold (i.e. if there is amplitude below the amplitude threshold <NUM>, this is regarded as exceeding an according amplitude change threshold). Thus, the interferogram resp. the measurement is classified as non-valid.

Instead of an amplitude value as a threshold <NUM> as shown, e.g. a threshold test based on a standard deviation of the measured amplitude chart <NUM> to an ideal amplitude chart is effected, comparable to the method as shown in <FIG>. As another option, not the curve <NUM> but its derivative is compared to a threshold, e.g. the gradient must not exceed a certain value.

In another procedure, illustrated by <FIG>, the classification <NUM> comprises not only a test if there is amplitude below an amplitude threshold (<NUM> in <FIG>) but additionally it is tested if such low amplitude section (B in <FIG>) accounts for a too big part of the whole interferogram. <FIG> is based on <FIG> whereby steps <NUM> and <NUM>' are not shown for reasons of more compact illustration.

As in <FIG>, in step <NUM>' any amplitude change is determined and in step <NUM>, it is verified if there is amplitude change above an amplitude threshold. If "no", the measurement is classified as a valid measurement (6b).

If there is change above the first threshold, then it is further evaluated if the fraction or portion of amplitude change above the first threshold is above a second threshold/fraction threshold (step <NUM>). Only if the second threshold is exceeded, the measurement is classified as non-valid (6a). Otherwise, the amplitude change, although above the amplitude threshold, is regarded as not rendering the measurement invalid (step 6b').

Said otherwise, it is not only looked if there is significant amplitude change but also if in the case of such major amplitude variation this change concerns at least a predefined portion of the interferogram. Only if high amplitude change is detected that prevails a defined period, then the interferogram is classified as non-valid (6a). Said the other way round, if amplitude change above the first threshold is present but the change lasts only a period shorter than a predefined period, the measurement is still regarded as valid as the amplitude change is significant but concerns only a portion of the interferogram which is regarded as a negligible portion.

With respect to <FIG>, the method according to <FIG> is further exemplified. In the example of <FIG>, region B shows amplitude below the amplitude or first threshold <NUM> as described above. Regions A and C show amplitude values above the first threshold <NUM>. Then, the fraction F of amplitude change above the first threshold <NUM> is calculated as <MAT>.

If the fraction F is above a defined fraction threshold, then the interferogram is classified as non-valid. In other words, if the size B amounts to more than a defined portion of the size of the whole interferogram, the interferogram is tagged as invalid.

The values of the described thresholds are e.g. determined by scanning tests on a typical rough surface. The described procedures are optionally combined to achieve higher robustness, e.g. there is test with respect to phase change and additionally test with respect to amplitude change. Both evaluations can be performed independently and if either one of both results in "non-valid", the measurement is classified as "non-valid". Or, these test are performed in sequence, for example first there is a test for amplitude change as described in <FIG>, and interferograms <NUM> classified as "non-valid" because of amplitude change are tested for phase change. If the phase change results in "non-valid" classification, too, the measurement is finally classified as "non-valid".

Otherwise, the measurement is classified as "valid". As an alternative, in case of divergent classification results with respect to amplitude and phase, amplitude and/or phase classification is done a second time, this time e.g. with more refined thresholds.

<FIG> illustrates a further development of the method of classification <NUM> of interferometric measurements. <FIG> is based on <FIG> or <FIG> whereby steps <NUM>, <NUM>/<NUM>' and <NUM>/<NUM>' are not shown for reasons of more compact illustration. In this further development, the detection of phase and/or amplitude disturbances in the interferograms is not only used to declare an interferogram, and also the resulting distance value, as invalid. It is also used to improve the robustness of "valid" distances, e.g. in case of (weaker) speckles disturbances.

In this further development according to <FIG>, after step <NUM> wherein the phase and/or amplitude change is used for classification of interferograms, a respective interferogram classified as non-valid (step 6a) is dismissed or ignored (step <NUM>). That means an interferogram recognized as non-valid is not used for calculation of a distance D to a point of the measured surface.

On the other hand, an interferogram classified "valid" is further processed in that the phase information is weighted, based on the amplitude of the respective interferogram (step <NUM>). This is particularly advantageous in classifications <NUM> that are based on amplitude change evaluation (e.g. as described with respect to <FIG>) as in these cases the amplitude is calculated/processed anyhow. With the time dependent weighting factor w(t) the phase information Φproc to be used for the distance processing is calculated from the raw phase information Φraw according to: <MAT>.

The weighted or processed phase information Φproc then is used for calculation of the distance D to the underlying surface point (step <NUM>).

Different weighting methods are applicable, whereby the weighting factor is preferably directly derived from the amplitude. As one option, the weighting factor is directly derived from the amplitude in that the amplitude itself is used as a weighting factor for the phase: <MAT>.

Another option for directly deriving the weighting factor from the amplitude is to use the power k of the amplitude as a weighting factor for the phase: <MAT> whereby k is e.g. a real number smaller or greater than <NUM>.

As a further option, phase information is only taken into account if the respective amplitude is above an amplitude threshold (e.g. threshold <NUM> in <FIG>). Said otherwise, phase information is excluded from distance processing where the amplitude is below the amplitude threshold.

For example, regions of "valid"-interferograms below the above described amplitude threshold (e.g. region B in <FIG>) are excluded from calculation of the distance (or only used with a lower power k than for the other amplitude regions, e.g. regions A and C in <FIG>). Said otherwise, e.g. in a classification <NUM> as shown in <FIG>, the gained knowledge about the fraction F/region B is used to eliminate (or at least diminish) the influence of the respective phase information on calculation of the distance to the surface point for "valid" interferograms showing such (temporarily minor) amplitude fluctuation.

The combination of this phase weighting method with the speckle detection for a whole measurement sweep is particularly advantageous. For this, strongly disturbed interferograms are tagged as "invalid" distances. Those distances can be excluded or interpolated like for example described with respect to <FIG>. The calculation of the remaining "valid" distances make use of one of the phase weighting methods described above which reduces the disturbing effect of weaker speckles. This approach drastically increases the overall robustness of the profile measurement.

Claim 1:
Method for, in particular industrial, measurement of a surface (<NUM>), in particular according to the principle of Optical Coherence Tomography, whereby based on interferograms (<NUM>) distances (D) to points of the surface (<NUM>) are measured, with
• generating a laser beam (<NUM>),
• emitting the laser beam (<NUM>) onto the surface (<NUM>),
• receiving at least a part of the laser beam (<NUM>), reflected by a respective point of the surface (<NUM>),
• generating an interferogram (<NUM>) by superposition of the received laser beam (<NUM>) with a reference laser beam,
the method further comprising classifying (6a, 6b, 6b') a respective interferogram (<NUM>) or the distance (D) derived therefrom as non-valid (6a) or valid (6b, 6b') based on evaluation (<NUM>) of amplitude change (<NUM>'), and in particular phase change (<NUM>), of the respective interferogram (<NUM>),
characterized in that
the evaluation (<NUM>) comprises detecting if the amplitude (A) of a respective interferogram (<NUM>) is temporarily below an amplitude threshold (<NUM>) and a respective measurement (<NUM>, <NUM>, <NUM>, <NUM>) is classified as non-valid (6a) if an interferogram fraction (F, B) with amplitude (A) below the amplitude threshold (<NUM>) is above a fraction threshold.