Patent Publication Number: US-11027364-B2

Title: Method and device for measuring the depth of the vapor capillary during a machining process with a high-energy beam

Description:
RELATED APPLICATION DATA 
     This application is a U.S. national stage of and claims priority benefit to prior filed international application no. PCT/EP2016/075112, filed Oct. 19, 2016, and which claims priority to German national application no. 10 2016 005 021.7, filed Apr. 22, 2016. The entire contents of these prior filed applications are hereby incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     1. Scope of the Invention 
     The invention relates to a method and a device for measuring the depth of the vapor capillary during a process in which workpieces are welded with a laser beam, an electron beam or another high-energy beam, provided with holes or processed in any other way. In particular, the invention relates to the mathematical evaluation of the measurement data generated by an optical coherence tomograph and superimposed by interferences. 
     2. Description of the Prior Art 
     Laser processing devices usually comprise a laser radiation source, which may be, for example, a fiber laser or a disk laser, and a machining head that focuses the laser beam generated by the laser beam source in a focal spot. The machining head may be attached to a movable robotic arm or other traveling device that allows positioning in all three spatial directions. Sometimes the machining head is fixed in space, and the workpieces are delivered by means of a handling device. 
     A problem that was previously unsatisfactorily solved when welding or drilling with the aid of laser beams, was keeping the penetration depth of the laser beam at the desired setpoint as accurately as possible. The penetration depth is the axial extent of the vapor capillary that is generated by the laser beam in the workpiece. Only when the penetration depth reaches its setpoint may the desired machining result be obtained. For example, if the penetration depth is not deep enough when welding two metal sheets, there is no, or only incomplete, welding of the two sheets. On the other hand, if the penetration depth is too deep, this may lead to welding through. 
     Undesirable variations in the penetration depth may occur for different reasons. For example, in the course of laser machining, the protective screen, which protects the optical elements in the machining head from splashes and other dirt, absorbs an increasing part of the laser radiation, wherein the penetration depth decreases. In addition, inhomogeneities in the workpieces or variations in the travel speed may cause the penetration depth to change locally and thus deviate from its setpoint. 
     The measurement of the depth of the vapor capillary presents a significant challenge because very difficult measuring conditions prevail within the vapor capillary. The vapor capillary is not only very small and extremely thermally bright, it also generally changes its shape during machining. 
     Comparable problems also arise when machining workpieces with electron beams or other high-energy beams. 
     EP 1 977 850 A1, DE 102010016862 B3, US 2012/0138586 A1 and US 2016/0039045 A1 describe methods in which the penetration depth of the laser beam during laser machining is measured with the aid of an optical coherence tomograph (OCT). Optical coherence tomography enables a highly accurate and contactless optical distance measurement even in the vicinity of the thermally very bright vapor capillary. 
     WO 2015/039741 A1 discloses a method, which is optimised especially for the measurement of the penetration depth. An optical coherence tomograph generates a first measuring beam which is directed to the bottom of the vapor capillary. At the same time, a second measuring beam is directed to a second measuring point which is located on the workpiece outside the vapor capillary. Preferably, this second measuring beam scans the surface of the workpiece like a scanner. The penetration depth of the laser beam then results as the difference between the distances measured with the aid of the two measuring beams. 
     One problem with such measurements is that the coherence tomographs provide a lot of measurement data from which the desired information must be filtered out. Significant problems arise in this case with respect to interferences, which partially cover the actually desired measurement signals. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to provide a method and a device with which the depth of the vapor capillary may be reliably and accurately determined during a machining process with a high-energy beam despite interferences. 
     A method according to the invention that achieves this object comprises the following steps: 
     a) directing an optical measuring beam to the bottom of a vapor capillary, which results in a region of interaction between a workpiece and the high energy beam; 
     b) detecting reflections of the measuring beam in an optical coherence tomograph; 
     c) generating raw measurement data from the reflections detected in the optical coherence tomograph, 
     d) repeating steps a) to c) at multiple times t i , where i=1,2,3, . . . , during the machining process, wherein a set of raw measurement data for a first distance to the bottom of the vapor capillary are obtained for each time t i ; 
     e) calculating a set of undisturbed measurement data for a time t n  by generating a first set of raw measurement data generated at the time t n , and a second set of raw measurement data generated at an earlier time t m , where m&lt;n, were processed together by means of a mathematical operation; 
     f) calculating a final value for the first distance at the time t n  from the set of undisturbed measurement data calculated in step e); 
     g) measuring a second distance to a part of the surface of the workpiece that is not exposed to the high energy beam; 
     h) calculating the depth of the vapor capillary by subtracting the second distance from the first distance final value calculated in step f). 
     The inventors realised that common mathematical processing of the current raw measurement data with raw measurement data that was generated at an earlier time, interferences that typically affect the measurement of the depth of the vapor capillary may be largely eliminated. This is based on the knowledge that the interfering influences usually do not change, or change only comparatively slowly, during the machining process. As a result, the interference between two successive measurement times has practically the same effect on the raw measurement data. With suitable mathematical operations, e.g. a complete or partial subtraction, then the unwanted interference may be largely eliminated. If the raw measurement data is present as spectra, then this mathematical operation may also be a division. 
     In one embodiment, the set of raw measurement data generated since the earlier time t m  is at least partially subtracted from the set of raw measurement data generated at time t n  by the mathematical operation in step e). For example, in the case of the coherence tomographs preferred here, wherein the distance values are coded in the spectrum of the reflected light (Spectral Domain, SD OCT), the measured spectra may be subtracted from one another. 
     It is often favorable in the case of a subtraction, if an immediately preceding time t n-1  is selected as the earlier time. Since the measurements are usually clocked with a given measurement frequency, there is an immediately preceding time t n-1  (except for the first time t o ) at each time t n . Such a choice is advantageous because it minimises the probability that the influence of the disturbance on the raw measurement data will have changed between the immediately adjacent times t n  and t n-1 . 
     In principle, it is possible that the raw measurement data generated at the earlier time t m  is not completely, but only partially, e.g. 99.9%, subtracted. In this way, for example, weakening interference may be taken into account during the measurement. This corresponds to the multiplication of the raw measurement data with a factor IgI&lt;1. As a complete subtraction, a subtraction is also considered here, wherein the raw measurement data generated at the earlier time are previously multiplied by a factor IgI&gt;1, in order to into take account an increasing interference in the course of the measurement. The factor g may also change in the course of the machining process. 
     In a preferred embodiment, the mathematical operation in step e) at least partially subtracts from the set of raw measurement data generated at time t n , a moving average calculated from sets of raw measurement data that were generated at several earlier times t j , where j≤m. In this way, the effect of faster-changing interferences may be effectively eliminated, since a trend in the course of the interference results from the change in the moving average. 
     The moving average may be a weighted average of at least the order 2. Orders of 3 or more are usually not required. 
     The average may also be an exponentially smoothed average that captures all previous times with progressively weaker weight. This makes it even more effective in the elimination of the influence of faster-changing interferences. 
     In the case of the coherence tomograph used, the distance values are preferably coded in the course of the interferometer phase as a function of the frequency of the reflected light. Such coherence tomographs are commonly referred to as FD OCT, wherein FD stands for Fourier Domain. 
     This type of coherence tomograph also includes the above-mentioned SD OCT, in which the invention may be used particularly advantageously. The set of raw measurement data is represented in such a coherence tomograph by an interference spectrum generated by the optical coherence tomograph. The distance values result from the inverse Fourier transforms of the (usually previously equalised) spectra. 
     Alternatively, the coherence tomograph may also be designed as a swept-source coherence tomograph (SS OCT), which also belongs to the FD OCT group. In an SS OCT, the wavelength of a narrowband light source is quickly tuned. Thus, an SS OCT does not require a spectrograph, but only a single photosensitive element that sequentially captures the spectral components. The spectral components may be joined to form an interference spectrum as generated by an SD OCT. 
     In particular, when the raw measurement data are spectra, in the mathematical operation in step e), the interference spectrum generated by the FD coherence tomograph at time t n  may be divided by an average spectrum which is an average of several interference spectra, which were generated at earlier times t j , where j≤m. Such a division performed before the inverse Fourier transform also effectively eliminates the influence of slowly varying interferences. By averaging, there is a smearing of the high-frequency components. What remains is only the low-frequency interference component, which divides the currently measured interference spectrum. 
     The more interference spectra that contribute to the average value, the better is the smearing of the high-frequency components. It is therefore favorable if the interference spectra contribute to the moving average of 50 to 200 earlier times. 
     According to the method of the invention, the interference spectra may be subjected to an inverse Fourier transform in a manner known per se. In this way, at least one distance value is obtained for each interference spectrum. From several distance values, the final value for the first distance at time t n  is then calculated according to a predetermined criterion. 
     For example, this criterion may be a quantile criterion. A quantile is a threshold that has the property that a certain proportion of the values is smaller than this value while the remaining proportion of the values is greater than this threshold. In particular, for the measurement of the vapor capillary, such a quantile filter has proven to be suitable, since only the largest distance values correctly reproduce the distance to the bottom of the vapor capillary. 
     In principle, the second distance to the surface of the workpiece may be determined with any measurement method. Tactile measurements as well as contactless measurements with the help of sound or electromagnetic waves are possible. 
     However, it is particularly simple if, in step g), the second distance is measured by directing another measuring beam onto that part of the surface of the workpiece which is not exposed to the high-energy beam. Reflections from the other measuring beam are then detected in the same or another coherence tomograph. The raw measurement data for the second distance may be processed in the same way as the raw measurement data for the first distance. In many cases, however, it is better to process the raw measurement data for the two distances in different ways. In the determination of the first distance, for example, a quantile filter may be used, while the second distance may be derived from parameters of a fitted distribution function. 
     In principle, it is possible to provide two independent coherence tomographs in order to measure the two distances independently. However, it is simpler if an original measuring beam is split into the measuring beam and the further measuring beam. When evaluating the raw measurement data, both values are then obtained simultaneously for the first distance and for the second distance. For the distribution of the original measuring beam, any optical elements may be used, which make it possible to spatially divide an incident light beam. Such an optical element may be, for example, designed as a polarisation-selective and non-polarisation-selective beam splitter. In the simplest case, a prism is used which has two mutually-inclined optical surfaces. If the original measuring beam is directed onto the prism so that it strikes both optical surfaces, the original measuring beam is refracted differently at the optical surfaces and is divided. If such a prism is rotated or moved in any other way and arranged so that a surface of the prism does not change its orientation during movement, it may be achieved that a measuring beam with fixed direction remains directed into the vapor capillary, while the other measuring beam scans the surface of the workpiece outside the vapor capillary. 
     With regard to the device, the object stated at the outset is achieved by a device for measuring the depth of the vapor capillary during a machining process with a high-energy beam, wherein the device comprises:
         an optical coherence tomograph that is designed to perform the following steps:
           a) directing an optical measuring beam to the bottom of a vapor capillary, which results in a region of interaction between a workpiece and the high energy beam;   b) detecting reflections of the measuring beam in the optical coherence tomograph;   c) generating raw measurement data from the reflections detected in the optical coherence tomograph,   d) repeating steps a) to c) at several times t i , where i=1, 2, 3, . . . , during the machining process, wherein for each time t i  a set of raw measurement data is obtained for a first distance to the bottom of the vapor capillary;   
           an evaluation device that is set up to carry out the following steps:
           e) calculating a set of undisturbed measurement data for a time t n  by generating a first set of raw measurement data at the time t n  and a second set of raw measurement data generated at an earlier time t m , where m&lt;n, wherein they were processed together by means of a mathematical operation;   f) calculating a final value for the first distance at time t n  from the set of undisturbed measurement data calculated in step e);   g) calculating the depth of the vapor capillary by subtracting a measured second distance to a part of the surface of the workpiece not exposed to the high energy beam from the first distance end value calculated in step f):   
               

     The advantageous embodiments explained for the method are correspondingly applicable to the device according to the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will be explained in more detail with reference to the drawings, wherein: 
         FIG. 1  shows a schematic representation of a laser machining apparatus according to the invention when welding two workpieces; 
         FIG. 2  shows the internal structure of the laser processing apparatus shown in a schematic representation in  FIG. 1 ; 
         FIGS. 3 a  and 3 b    show a sectional view through a wedge plate, with the aid of which two measuring beams are generated in two different rotational positions; 
         FIG. 4  shows an enlarged section of two workpieces, in which the vapor capillary is visible; 
         FIG. 5  shows a plan view of the detail shown in  FIG. 4 ; 
         FIG. 6  shows a graph schematically illustrating the generation of spectra at optical interfaces in an FD coherence tomograph; 
         FIG. 7  shows a graph in which measured distance values versus time are shown during a welding run; 
         FIG. 8  shows a graph in which two spectra, which were generated by two different boundary surfaces in the beam path of the measuring light, are shown to explain a first exemplary embodiment; 
         FIG. 9  shows a graph showing the inverse Fourier transforms of the spectra shown in  FIG. 8 ; 
         FIG. 10  shows a graph in which a difference spectrum between two interfered spectra is shown; 
         FIG. 11  shows a graph showing the inverse Fourier transform of the difference spectrum shown in  FIG. 10 ; 
         FIG. 12  shows a graph showing an uninterfered spectrum; 
         FIG. 13  shows a graph showing the inverse Fourier transform of the uninterfered spectrum shown in  FIG. 12 ; 
         FIG. 14  shows a graph showing an interfered spectrum; 
         FIG. 15  shows a graph showing the inverse Fourier transform of the interfered spectrum shown in  FIG. 14 ; 
         FIG. 16  shows a graph showing a spectrum resulting from the additive superposition of two spectra interfered by a ripple; 
         FIG. 17  shows a histogram of a typical frequency distribution of distance values obtained over a given period of time; 
         FIGS. 18 a  and 18 b    show graphs in which distance values obtained from disturbed and undisturbed, respectively, spectra and distances calculated by quantile filtering, are plotted over time t. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     1. Structure of the Laser Machining Apparatus 
       FIG. 1  shows a schematic representation of an exemplary embodiment of a laser machining apparatus  10  according to the invention, wherein it comprises a robot  12  and a machining head  14  which is fastened to a movable arm  16  of the robot  12 . 
     The laser machining apparatus  10  also includes a laser radiation source  18 , which is in the form of a disk laser or fiber laser in the illustrated embodiment. The laser beam  19  generated by the laser radiation source  18  is supplied via an optical fiber  20  to the machining head  14  and focused by the latter in a focal spot  22 . 
     In the illustrated embodiment, the laser machining apparatus  10  is to be used to weld a first metallic workpiece  24  of varying thickness to a second metallic workpiece  26  mounted on a workpiece holder  27 . The focal spot  22  produced by the machining head  14  therefore has to be precisely positioned in the vicinity of the transition between the first workpiece  24  and the second workpiece  26 . 
       FIG. 2  shows a schematic representation of the internal structure of the optical components of the laser machining apparatus  10 . The laser beam  19  generated by the laser radiation source  18  emerges from the optical fiber  20  in the machining head  14  and is collimated by a first collimator lens  28 . The collimated laser beam  19  is then deflected through 90° by a dichroic mirror  30  and impinges on a focusing optics  32 , the focal length of which may be changed by axially displacing one or more lenses by means of an actuator  34 . In this way, the axial position of the focal spot  22  may be changed by adjusting the focusing optics  32 . The last optical element in the beam path of the laser beam  19  is a protective screen  38 , which is exchangeably fixed to the machining head  14  and protects the remaining optical elements from splashes and other contaminants, which arise at the machining point indicated at  36 . 
     The laser machining apparatus  10  also includes an optical coherence tomograph  40  (so-called SD OCT, Spectral Domain Coherence Tomograph) operating in the spectral range. The coherence tomograph  40  has a light source  42 , an optical circulator  44  and a fiber coupler  46 , which divides the measurement light  48  generated by the light source  42  into a reference arm  50  and an object arm  52 . In the reference arm  50 , after passing through an optical path which corresponds approximately to the optical path of the measurement light in the object arm  52 , the measurement light is reflected by a mirror  53  and returns to the optical circulator  44 , which forwards the measurement light to a spectrograph  54 . 
     In the object arm  52 , the measuring light emerges at the end of another optical fiber  56  and is collimated by a second collimator lens  58 . The collimated measuring light  48  first passes through a first Faraday rotator  86 , which rotates the polarisation direction through 45°. A similar second Faraday rotator  84  is arranged in the section of the free beam propagation in the reference arm  50 . The two Faraday rotators  84 ,  86  have the task of avoiding interference that may arise when the optical fibers used in the coherence tomograph  40  do not receive the polarisation state. 
     Subsequently, the collimated measuring light  48  impinges on a wedge plate  60 , which may be rotated by a motor  62  about an axis of rotation  64 . As may be seen in the enlarged view of  FIG. 3 a   , the wedge plate  60  has a first flat surface  66  that is aligned perpendicularly to the axis of rotation  64  and provided with a coating  68  which reflects about 50% of the incident measuring light  48 . Since the flat surface  66  does not change its orientation upon rotation of the wedge plate  60 , it produces a first measuring beam  70   a  whose direction is also invariable. 
     The portion of the measuring light  48 , which passes through the partially reflecting coating  68 , strikes a second flat surface  72  of the wedge plate  60 , which is at an angle different from 90° to the axis of rotation  64 . The orientation of the second flat surface  72  thus depends on the angle of rotation of the wedge plate  60 . The second flat surface  72  is provided with a completely reflective coating  74 . Since the two planar surfaces  66 ,  72  are not parallel to one another, the second flat surface  72  generates a second measuring beam  70   b , which has a different propagation direction to that the first measuring beam  70   a . The direction of propagation depends on the angle of rotation of the wedge plate  60  with respect to the axis of rotation  64  as illustrated in  FIG. 3 b   . In this case, the wedge plate  60  has been rotated about the axis of rotation  64  through an angle of 180° compared to the arrangement shown in  FIG. 3 a   . When the wedge plate  60  rotates about the axis of rotation  64 , the second measuring beam  70   b  therefore continuously rotates about the stationary first measuring beam  70   a.    
     A similar effect may also be achieved with a rotating transmission prism having an inner circular-shaped region whose parallel flat surfaces are oriented perpendicularly to the axis of rotation. This region is surrounded by a ring cut out of a wedge. During the rotation of the prism by means of a hollow shaft, at least one wedge surface of the ring changes its orientation. Measuring light  48  falling on the inner circular disk-shaped region is not interrupted and forms the first measuring beam  70   a . The light incident on the ring is deflected at the inclined wedge surface to form the second measuring beam  70   b , which rotates about the fixed first measuring beam  70   a . Reference is again made below to  FIG. 2  in order to explain the beam path of the two measuring beams  70   a ,  70   b  in more detail. The measuring beams  70   a ,  70   b , which are indicated by solid or double-dashed lines, are first expanded by means of a diverging lens  76  and then collimated by a third collimator lens  78 . After passing through the dichroic mirror  30 , which is transparent to the wavelengths of the measuring light, the measuring beams  70   a ,  70   b  are focused exactly as the laser beam  19  of the focusing optics  32  and directed to the workpieces  24 ,  26  after passing through the protective screen  38 . Since the first measuring beam  70   a  propagates coaxially with the laser beam  19 , the focal spot  80  of the first measuring beam  70   a  coincides with the focal spot  22  of the laser beam  19 . The focal plane of the second measuring beam  70   b  is coplanar with the focal plane of the laser beam  19  and the first measuring beam  70   a.    
     The conditions at the machining point  36  will be described in more detail below with reference to  FIG. 4 .  FIG. 4  shows an enlarged detail of the workpieces  24 ,  26  which are to be welded together. The direction of travel of the machining head  14  relative to the workpieces  24 ,  26  is designated by  98 . 
     The focused laser beam  19  emerging from the protective screen  38  reaches such a high energy density in the vicinity of the focal spot  22  that the surrounding metal evaporates to form a vapor capillary  88 , which extends into the two workpieces  24 ,  26 . Although a portion of the vaporised metal forms a cloud  90  over the surface  91  of the first workpiece  24 , the vapor capillary  88  only refers to the cavity forming beneath the surface  91  during machining. 
     The vapor capillary  88  is surrounded by a melt  92 , which solidifies with increasing distance from the focal spot  22  of the laser beam  19 . In the region of the melt  92 , the materials of the two workpieces  24 ,  26  join together. When the melt  92  solidifies, this results in a weld  94 , whose upwardly-facing side is irregularly wavy and is referred to as a weld bead  96 . 
     In the enlarged view of  FIG. 4 , it may be seen that the focal spot, which is generated by the first measuring beam  70   a , approximately coincides with the focal spot  22  of the laser beam  19 . In the vicinity of the focal spot  22 , the first measuring beam  70   a  strikes the metallic melt  92  at the bottom of the vapor capillary  88  and is reflected back therefrom into the object arm  52  of the coherence tomograph  40 . The point at which the first measuring beam  70   a  meets the bottom of the vapor capillary represents a first measuring point MPa assigned to the first measuring beam  70   a.    
     The point at which the second measuring beam  70   b  is reflected by the surface  91  of the first workpiece  24  surrounding the vapor capillary  88  represents a second measuring point MPb assigned to the second measuring beam  70   b.    
       FIG. 5  shows a plan view of the first workpiece  24  for the section shown in  FIG. 4 . If the machining head  14  is moved in the travel direction  98  to produce a weld  94 , then the previously mentioned weld bead  96  is formed in the travel direction  98  behind the vapor capillary  88 . An arrow  100  indicates how the second measuring point MPb rotates upon rotation of the wedge plate  60  on a circular path  102  around the processing point  36 . The second measuring point MPb also passes over part of the melt  92 . If the wedge angle of the wedge plate  60  is selected to be larger, the radius of the circle  102  increases. In this case, the second measuring point MPb may also sweep the welding bead  96 . At a measuring frequency of the coherence tomograph  40  in the order of a few kHz, a rotation frequency of the wedge plate  60  in the order of 100 Hz and a speed along the direction of travel  98  in the order of 1 to 10 m/min, the relief of the surface  91  in the vicinity of the processing station  36  may be scanned with high resolution in this way. 
     2. Determination of the Penetration Depth 
     The penetration depth is designated d in  FIG. 4  and is defined as the depth of the vapor capillary  88  below the surrounding (still solid) surface  91  of the first workpiece  94 . If the penetration depth is not deep enough, the two workpieces  24 ,  26  are not, or only partially, welded together. On the other hand, if the penetration depth d is too deep, through welding occurs. 
     For flat workpieces of constant thickness, the penetration depth d is often constant. In general, however, the penetration depth d depends on the coordinates x, y on the workpieces. Changes in the penetration depth d may be required, for example, if the thickness of the first workpiece  24  is location-dependent. 
     For measuring the penetration depth d, the first measuring beam  70   a  measures at the first measuring point MPa, the distance of the bottom of the vapor capillary  88  relative to a reference point at which this is possible, for example a point on the surface of the protective glass  38  that is traversed by the optical axis OA. In  FIG. 4 , this distance is designated a1. 
     At the second measuring point MPb, the second measuring beam  70   b  measures the distance between the reference point and the surface  91  of the first workpiece  24  surrounding the vapor capillary  88 . The penetration depth d then results simply as the difference between the distances a2 and a1. 
     The measuring light that was guided in the object arm  52  and, after reflection at the measuring points MPa, MPb, that entered the object arm  52  again, is evaluated in order to determine the distances a2 and a1. This portion of the measuring light passes through the further optical fiber  56  back to the fiber coupler  46  and the optical circulator  44  and interferes in the spectrograph  54  with the measuring light which has been reflected in the reference arm  50 . The interference signal is fed to a control and evaluation device  114  (see  FIG. 2 ), which then calculates the optical path length difference of the measurement light reflected in the reference arm  50  and in the object arm  52 . From this, the distances a1, a2 of the measuring points MPa, MPb may be derived from the common reference point. 
     In that regard, the construction and the function of the laser machining device  10  are already known from the aforementioned WO 2015/039741 A1. What is new and inventive is the procedure described below, wherein the measurement data of the coherence tomograph are computationally evaluated by the control and evaluation device  114 . 
     3. Computational Evaluation of the Measurement Data 
     As mentioned above, the coherence tomograph  40  is an SD OCT using a comparatively broad band light source  42 . All the reflected spectral components of the measuring light are detected simultaneously in the spectrograph  54 . Such coherence tomographs make it possible to determine a complete depth profile of partially reflecting or scattering structures in a single measurement. However, the coherence tomograph  40  may also be embodied as SS OCT (Swept Source OCT), in which the wavelength of a narrow band light source is quickly tuned in. In this case, a single light-sensitive element which sequentially determines the spectral components is sufficient. However, in this case, significantly fewer distance values are received, so that the rapidly-changing distance a1 to the bottom of the vapor capillary  88  may not be measured frequently enough. 
     In the following, therefore, an SD OCT is assumed; the following remarks apply accordingly to SS OCT mutatis mutandis. 
     In an SD OCT, at any time t i , i=1, 2, 3, . . . , during the laser machining process, a set of raw measurement data in the form of an interference spectrum is obtained. The spectral intensity P int (k) of the measuring light detected by the spectrograph  54  is described by the equation (1): 
                       P   int     ⁡     (   k   )       =         P   ein     ⁡     (   k   )       ⁢       {       R   R     +       ∑   j     ⁢     R     S   ,   j         +     2   ⁢       ∑   j     ⁢           R   R     ⁢     R     S   ,   j           ⁢     cos   ⁡     (     2   ⁢           ⁢     k   ⁡     (       z   R     -     z     s   ,   j         )         )             +     2   ⁢       ∑     j   ,   m               ⁢           R     S   ,   j       ⁢     R     S   ,   m           ⁢     cos   ⁡     (     2   ⁢     k   ⁡     (       z     s   ,   j       -     z     s   ,   m         )         )               }     .               Eq   .           ⁢     (   1   )                 
Where P int (k) is the power spectrum of the light source  42 , R R  is the reflectance in the reference arm, R Sj  is the reflectance of the j th  interface or structure in the measurement object, z R  is the optical path length in the reference arm  50 , and z Sj  is the optical path length in the object arm  52  to the j th  interface or structure.
 
     The first summand P int (k)Σ j R s,j  describes a DC component that depends on the power spectrum of the light source  42  and the reflectance R R  in the reference arm  50 . This proportion may be determined by implementing a measurement without a measurement object. It then holds for all reflectivities R Sj =0, where P int (k)=P ein (k) R R . Therefore, this proportion is hereinafter referred to as “dark spectrum”. 
     The second summand P ein (k)Σ j R s,j  describes another DC component, which depends on the power spectrum of the light source  42  and the reflectance RSj of the structures in the object arm  52 . If an ideal mirror with the highest possible spectrally independent reflectance is used as the measurement object, then Σ j R s,j ≈1. From a measurement of P int (k), the power spectrum P ein (k) of the light source  42  may then be determined with knowledge of the dark spectrum, which is referred to here as the “white spectrum”. 
     The term in the second line contains cross-correlations that are of interest for the measurement. Every reflection in the measurement object leads to a modulation of the interference spectrum in k-space. In other words, each frequency component in the measured interference spectrum corresponds to a specific distance from a partially reflecting or scattering structure of the measurement object. Since the length of the reference arm  50  is usually chosen so that it is either significantly shorter or significantly longer than all typically occurring optical path lengths in the object arm  52 , each modulation frequency may be assigned a unique distance ZSj in the measurement object. 
     The term in the third line of equation (1) describes the autocorrelation of the measurement object, which is not due to the interference between the reflections on the measurement object and the reference arm, but rather to the interference with one another of the reflections on the measurement object. Since the reflectivity in the reference arm R R  is usually much larger than the reflectivities R Sj  in the measurement object, the third term is negligible compared to the second term in most cases. 
       FIG. 6  illustrates these relationships in a schematic representation. On the left is shown an arrangement of two glass plates  104 ,  106 . At each of the three optical interfaces  1081 ,  1082  and  1083 , there is a jump in refractive index and thus a partial reflection of the incident measurement light  48 . 
     Three interference spectra  1 ,  2 ,  3  are shown to the right of the arrangement of the glass plates  104 ,  106 . The interference spectrum  1  is detected by the spectrometer  54  when only the first interface  1081  is in the beam path of the measuring light  48 . The reflection of the measuring light  48  at the first interface  1081  leads, according to the term in the second line of equation (1), to a modulation of the spectrum in k-space, which is proportional to the difference of the optical path lengths of the measuring light  48  in the reference arm  50 , on the one hand, and in the object arm  52 , on the other hand. The sought distance information is thus encoded in the frequency with which the intensity oscillates in k-space. 
     Corresponding references apply to the interference spectra  2  and  3  and the optical interfaces  1082  and  1083  assigned to these interference spectra. The optical path length in the reference arm  50  is determined here so that the modulation of the intensity in the k-space is all the higher, the more remote is the optical interface from the coherence tomograph  40 . 
     However, since the measuring light  48  strikes not only one of the interfaces  1081 ,  1082 ,  1083  but all interfaces, the interference spectra  1 ,  2  and  3  are superimposed. The spectrometer  54  thus determines only the complete spectrum  110  shown on the right, which represents an additive superimposition of the interference spectra  1 ,  2  and  3 . 
     The spectral components, i.e. the modulation frequencies of spectra  1 ,  2  and  3 , are obtained by an inverse Fourier transform. This is shown on the right in  FIG. 6 . In the example shown, a distance z is obtained which is assigned to the interference spectra  1 ,  2  and  3  for each optical interface  1081 ,  1082  and  1083 . 
     However, the representation of the inverse Fourier transforms is greatly simplified in  FIG. 6 . As already mentioned, the overall spectrum  110  measured by the spectrometer  154  is not only supported by the individual spectra of interest  1 ,  2  and  3 , but also by the constant DC contribution from the first line of equation (1) and the cross-correlation between the portions of the measuring light  48  according to the third line of the equation (1). The inverse Fourier transform therefore contains further contributions, which are not shown on the right in  FIG. 6  for the sake of simplicity. In the equation (2) in which the inverse Fourier transform of the spectral intensity P int (k) is given, these other contributions are listed in the first and third lines. 
     
       
         
           
             
               
                 
                   
                     
                       
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     From the above explanation, it is clear that as a rule, not only a single but a multiplicity of distance values z is obtained at any time t i . Any structure that causes a portion of the incident measuring light to go back again into the object arm  52  of the coherence tomograph  40  due to reflection or scattering, thus leads to a distance value z. 
     In  FIG. 7 , real measured values are plotted for the distance z obtained by Fourier transform, wherein they were determined during a welding run with the aid of the coherence tomograph  40 . In this case, a coherence tomograph was used in which the first measuring beam  70   a  propagates coaxially with the laser beam  19 ; wherein the distance to the surface  91  was not determined by a second measuring beam  70   b , but by a mechanical tactile sensor. Thus, the graph contains only measurement data for the distance a1 during the laser machining. 
     At time t 1 , the first measuring beam  70   a , and thus also the laser beam  19 , strike the first workpiece  24 . The measured value z=z w  initially corresponds to the distance a2 to the surface  91 . Shortly thereafter, i.e. at time t 2 , the temperature in the workpiece  24  is so high that the vapor capillary  88  is formed. It may be seen that the distance values z are now scattered over a large distance range. Experiments have shown that the first measuring beam  70   a  is often reflected before reaching the bottom of the vapor capillary  88 . The exact causes of this are not yet known in detail, since the processes in the vapor capillary  88  are complex and difficult to observe. Possibly, the vapor capillary  88  moves so rapidly in the lateral direction during laser machining that the first measuring beam  70   a  often only strikes the lateral wall of the vapor capillary, but not its bottom. Metal oxide droplets which form in the vapor capillary  88  by condensation of the metal vapor or by dissolving splashes from the melt  92  are also conceivable as the cause. 
     Investigations have shown that only the largest of the widely scattered distance values represent the distance a1 to the bottom of the vapor capillary  88 . These largest distance values may be determined by using special filters, e.g. like the quantile filter as explained below in more detail. 
     At time t 2 , the laser radiation source  18  is turned off and the direction of movement is reversed. The first workpiece  24  is then moved so that the first measuring beam  70   a  moves off the weld bead  96  that has formed after cooling of the metallic melt  92 , and detects its relatively rough surface profile  111 . At time t 4 , the first measuring beam  70   a  again reaches the end of the first workpiece  24 . 
     In the graph of  FIG. 7 , additional artifacts, which affect the above-described measurement as interfering signals are recognisable. A first artifact  112  appears to be at a distance of about z=2.9 mm and is represented by a number of measurements approximately on a horizontal line, thereby indicating a stationary scattering or reflecting interface. Even during the formation of the vapor capillary  88 , such an interface appears to be located above the vapor capillary, whereas on the return of the first measurement beam  70   a  via the weld bead  96  (t&gt;t 3 ), this apparent interface is no longer present. 
     Several other artifacts are located at a distance of less than 1 mm; here, too, most of the measuring points are lined up along horizontal lines and suggest the existence of stationary reflecting or scattering surfaces in the said distance range. 
     The causes of these artifacts are manifold. Investigations by the Applicant have shown that the measured distance values also contribute to very weak (multiple) reflections which arise in optical fibers and, in particular, at their connectors. As a result of the high measuring sensitivity, the coherence tomograph  40  also detects such extremely weak reflections. 
     Another cause of artifacts is probably due to comparatively slow changes of optical components in the beam path of the measuring light. These changes may cause the above-mentioned dark spectrum, which is generally subtracted from the measured interference spectrum. However, since the dark spectrum is detected only once before the measurement process and then subtracted unchanged from the measured interference spectra during the entire measurement process, slow drift movements may lead to the artifacts shown. 
     In the following, different approaches are described of how to effectively suppress the measurement data generated with the aid of the coherence tomograph  40  in order to obtain more accurate measured values for the depth d of the vapor capillary  88 . 
     a) Subtraction of the Predecessor Spectrum 
       FIG. 8  shows a graph in which, by way of example, two interference spectra, which were generated by two different boundary surfaces in the beam path of the measuring light, are plotted. The abscissa shows the pixel number p as a function of the wave number k. The pixel number p refers to the pixels of the CCD sensor contained in the spectrograph  54  and recording the interference spectra. Each pixel corresponds to a certain wave number k, wherein the relationship is not necessarily linear. Therefore, the pixel number p must generally be converted into the wave number k by using a suitable transformation function. 
     A solid line in  FIG. 8  shows a first interference spectrum  121  while a dashed line shows a second interference spectrum  122 . Each of the interference spectra corresponds to a partially reflecting or scattering optical structure and thus a certain distance value z. The two interference spectra  121 ,  122  are additionally modulated by a slowly varying interference spectrum  123 , which represents an interference. In order to better distinguish the two interference spectra  121 ,  122 , they are not in superimposed form as they are actually detected by the spectrograph  54  and applied separately. 
       FIG. 9  shows a graph in which the Fourier transforms  121 ′,  122 ′ of the interference spectrums  121 ,  122  modulated by the interference, and are plotted separately so that they may be better distinguished from one another. Since the disturbed interference spectrum  121  may be described as the product of an undisturbed interference spectrum and the interference spectrum  123 , the Fourier transform  121 ′ of the disturbed spectrum  121  results in a convolution of the Fourier transform of the undisturbed spectrum with the Fourier transform of the interference spectrum  123 . 
     In the graph of  FIG. 9 , it may be seen that the solid-lined Fourier transform  121 ′ of the disturbed interference spectrum  121  has two low-intensity distance peaks  121 ′ a  and  121 ′ b  at z=±20 mm around which are respectively arranged the symmetrical two stronger-intensity interference distance peaks  123 ′ a  and  123 ′ b , which are a result of the convolution. The distance peaks  121 ′ a ,  121 ′ b  indicate the existence of a partially reflecting or scattering structure at a distance of 20 mm. Further interference distance peaks  123 ′ a ,  123 ′ b  are arranged symmetrically at z=0 mm. 
     Since the distance peaks  121 ′ a ,  121 ′ b  actually of interest are less powerful than the surrounding interference distance peaks  123 ′ a ,  123 ′ b , no filtering may be carried out in the sense that only the strongest intensity distance peaks in the Fourier transform are considered “genuine” distance values to be taken into account. Conversely, it is also not possible to ignore the strongest intensity distance peaks, since the ratios shown in  FIG. 9  may also be precisely exchanged in the case of less powerful interferences or stronger reflections at optical interfaces. i.e. the spacing peaks of interest may be more powerful than the spurious distance peaks. 
     Corresponding considerations also apply to the Fourier transform of the disturbed second interference spectrum  122 ′. In this case, also, there are interference distance peaks  123 ′ 1 ,  123 ′ b  symmetrical around the distance peaks  122 ′ a ,  122 ′ b  of interest at z=±24 mm and around the central value z=0. Since the Fourier transforms  121 ′,  122 ′ in superimposed form result from the Fourier transform of the entire spectrum, it may be very difficult to filter out the desired distance information from the Fourier transform of the entire spectrum. 
     To solve this problem, it is proposed according to a first embodiment of the invention, that each interference spectrum P int,tn (k), that was generated at time t n , the spectrum P int,tn-1  (k), that was generated at the immediately preceding instant t n-1 , are subtracted at least partially and preferably completely according to equation (3):
 
Δ P   int,tn ( k )= P   int,tn ( k )− P   int,tn-1 ( k )  Eq.3
 
     The Fourier transform is then supplied only for the thus calculated difference spectrum ΔP int,tn (k). 
       FIG. 10  illustrates this with reference to a graph in which a difference spectrum ΔP int,tn (k) is shown by way of example. It was assumed here that at the time t n-1 , a disturbed interference spectrum was obtained, which represents a distance value of z=24 mm, and at time t n , a disturbed interference spectrum was obtained which represents a distance value z=20 mm. Since the interference spectrum  123  does not change, or changes only insignificantly, between two immediately successive times t n , t n-1 , the influence of the interference spectrum in the subtraction of the disturbed interference spectra is largely eliminated. The difference spectrum ΔP int,tn (k) obtained by the subtraction is denoted by  124  in  FIG. 10  and represents a beat which oscillates with the average of the individual frequencies and is modulated with the beat frequency, which is given by the amount of the difference of the individual frequencies, 
       FIG. 11  shows the Fourier transform  124 ′ of the difference spectrum  124  in a representation similar to that of  FIG. 9 . This contains only the desired distance peaks at z=±24 mm and z=±20 mm. The influence of the interference was thus completely eliminated by the difference formation. For each measurement, not only one, but two distance peaks are obtained. In principle, it could be determined which distance peaks were added at the time tn at the last measurement by comparison with the previous measurement results. Since a large number of distance peaks occur anyway during the measurement, the multiple values obtained multiple times may simply be additionally taken into account in the subsequent evaluation without this impairing the measurement accuracy. 
     This procedure works all the better, the slower are the interference changes during the welding run. As already explained above, some interferences are largely constant. Other interferences change due to various drift events, but are very slow compared to the rapid fluctuations in the vapor capillary  88 . As a result, the difference formation described above may very well reduce the influence of the interferences in the measurement within a rapidly changing vapor capillary. 
     If it is known that the intensity of the interference spectrum  123  changes, and may be taken into account by the fact that the interference spectrum measured at the immediately preceding instant t n-1  is not completely, but only partially, for example 99.9%, subtracted. In this way, a weakening interference may be taken into account in the course of the measurement. Of course, more complicated dependencies are possible. In this case, from equation (4)
 
Δ P   int,tn ( k )= P   int,tn ( k )− P   int,tn-1 ( k )· g ( k )  Eq. 4
 
where the function g(k) f expresses the change in the interference. If the interference increases, it goes without saying that g(k)&gt;1.
 
     In principle, it is also possible not to subtract the interference spectrum generated at the immediately preceding time t n-1  but at a later time, e.g. t n-2  or t n-3 . However, this will usually only be practicable if the interference remains essentially constant during the measuring process. 
     In order to be able to eliminate the influence of more rapidly changing interferences, a moving average calculated from the perturbing interference spectrum measured at time t n  may be at least partially calculated from several perturbing interference spectra at several earlier times t j , where j&lt;n, were generated. By forming such a moving average, short-term changes in the interference spectrum may be effectively eliminated because the moving average identifies a trend in the course of the interference spectrum. The simple moving average is calculated according to equation (5): 
     
       
         
           
             
               
                 
                   
                     
                       
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     In many cases, averaging with the order m=2 is sufficient. 
     It is particularly favorable if an exponentially smoothed average value  P int,tn (k)′  according to equation (6)
 
   P   int,tn ( k )′ =α P   int,tn ( k )−(1−α)   P   int,tn-1 ′( k )   Eq. 6
 
is used. In this way, earlier spectra are weighted more heavily than more recent spectra, wherein the average value reacts very quickly to changes in the same smoothing.
 
     The concept of completely or partially subtracting a predecessor spectrum or an average of several predecessor spectra from the measured interference spectra, also has the advantage that the dark spectrum no longer has to be recorded before each welding run and subtracted from the measured interference spectra. These temporally largely immutable influences are automatically eliminated by the difference formation described above. 
     It is clear from  FIG. 9  that a difference formation after carrying out the Fourier transform no longer makes sense. If the Fourier transform of a predecessor spectrum were to be subtracted from the Fourier transform of a disturbed interference spectrum, this would result in the example shown in  FIG. 9  with a difference of the two Fourier transforms of the perturbed spectra designated  121 ′ and  122 ′. However, this difference differs significantly from that shown in  FIG. 11 . Ultimately, this is due to the fact that the Fourier transform causes the disturbing effects due to the folding multiplied in the Fourier space in such a way that they can no longer be eliminated by means of a simple mathematical operation. 
     b) Division by Predecessor Spectrum 
     In a second embodiment, another approach is taken to reduce the impact of interferences on the measurement. For this purpose, each interference spectrum P int,tn (k), which was generated at time t n , divided by an average interference spectrum  P (k), which is an average of a plurality of interference spectra, which were generated at earlier times t j , where j&lt;m. 
     To explain this second approach, reference is first made to  FIG. 12 , which shows an undisturbed interference spectrum  132  generated by the coherence tomograph  40  after being divided by the above-mentioned white spectrum. This eliminates the equation P ein (k) in equation (1), which otherwise leads to undesired widening of the distance peaks in the Fourier transform of the measured interference spectrum. In addition, the signal was windowed in a manner known per se with a cos 2  function  134  in order to reduce the smearing in Fourier space. Here, for the sake of simplicity, only the reflection at a single interface is considered. 
     Accordingly, the Fourier transform  130 ′ shown in  FIG. 13  is characterised by two narrow and distinct distance peaks  132 ′ a ,  132 ′ b  arranged symmetrically about z=0, and indicating the distance of the interface z=40 mm. 
       FIG. 14  shows a graph corresponding to  FIG. 12 , which shows a disturbed interference spectrum  132   g  (also after division by the abovementioned white spectrum and windowing). The interference is represented by multiplication with a interference function  136  which modulates a ripple on the undisturbed interference spectrum. This disturbance may be caused for example by fluctuations in the power spectrum P ein (k) of the light source  42  or by unwanted (multiple) reflections within the coherence tomograph  40 . 
     As a result of this interference, the Fourier transform  132   g ′ of the disturbed interference spectrum has a plurality of interference distance peaks  138 ′ a ,  138 ′ b  resulting from the convolution of the Fourier transform  132 ′ shown in  FIG. 13  with the Fourier transform of the interference function. In  FIG. 15 , the intensities of the disturbing distance peaks  136 ′ a ,  136 ′ b  are small; However, in the case of stronger interferences, these intensities may also be higher than the intensities of the distance peaks  132 ′ a ,  132 ′ b , so that under certain circumstances no simple differentiation from the distance peaks of interest is possible. 
     To explain how the disturbing distance peaks may be suppressed according to the second approach, reference is made to  FIG. 16 , which shows an interference spectrum resulting from the additive superimposition of two interference spectra disturbed by a ripple, which were determined by the coherence tomograph  40  at successive times t n  and t n-i . The two interference spectra correspond to very closely spaced distance values z=40 and z=40.8. It may be seen that these two spectra compensate each other in part, and even almost completely, at the beat node  138 , so that essentially only the ripple modulated as an interference remains. 
     As simulations have shown, this effect occurs all the more completely, the more that similar interference spectra are additively superimposed. With 50 superimposed interference spectra, the interference function  136  already appears very clearly. With a superimposition of 100 interference spectra, only the interference function remains, while virtually no high-frequency spectral components are any longer recognisable. The fast measurement signal fluctuations are averaged out by the addition of the interference spectra, while the slower fluctuations remain by, for example, going back to drifting. 
     If one divides the interference spectrum Pint, tn (k) measured at a time tn by an average formed according to equation (5) from a large number of previously acquired interference spectra, the order m should be very large (preferably 50≥m≥100 and in particular 100≥m≥500), and so the interference component will be very largely reduced. The result is almost ideal interference spectra, as shown in  FIG. 12 . Accordingly, the distances may be reliably derived from the Fourier transform, as shown in  FIG. 13 . The weak distance peaks associated with the vapor capillary  88  will then no longer be masked by more intense interference distance peaks. 
     In order that the sum signal is sufficiently “smeared” during the addition of the spectra, the phase position of the high-frequency spectral components should vary statistically. This requirement is usually present, since the surfaces of workpieces usually have a roughness of a few micrometers, resulting in greater variations of the phase position. Conversely, the interference function should change as little as possible during the period considered by the averaging, since otherwise it would also be averaged out. 
     Alternatively, this approach may also be described in such a way that the measured spectra are normalised by a specially defined and continuously updated white spectrum. In the conventional procedure, it may be the case that the white spectrum measured once in the course of the measurements deviates more and more from the actual power spectrum P ein (k) of the light source  42  and thus leads to measurement errors. The “entrainment” of the white spectrum by means of continuous averaging ensures that such changes in the power spectrum P ein (k) are automatically taken into account. 
     c) Filtering 
     By means of the approaches described above, it is possible to remove the artifacts shown in  FIG. 7 . However, there remains the problem of determining meaningful values for the distances a1 and a2 from the widely scattering distance values measured by the first measuring beam  70   a  in the region of the vapor capillary  88  and by the second measuring beam  70   b  on the surface  91  of the workpiece  24 . 
       FIG. 17  shows a histogram of a typical frequency distribution of distance values z obtained over a time period T in which a plurality of successive measurement times t n  are present. The maximum  140  at relatively large distance values z is comparatively wide and is based on measured values which originate from the first measurement beam  70   a  directed into the vapor capillary  88 . The narrower maximum  142  at smaller distance values z is due to measured values originating from the second measuring beam  70   b , which is directed onto the surface  92  of the workpiece  24 . In order to obtain the desired distances a1, a2 from such a distribution, the measured distance values must be subjected to additional filtering. Possible approaches to this will be described in more detail below. 
     i) Quantile Filter 
     Investigations have shown that only the largest distance values correctly reproduce the distance a1 to the bottom of the vapor capillary  88 . To determine these largest distance values, quantile filtering may be implemented. A quantile is a threshold that has the property wherein a certain proportion of the values is less than this threshold, while the remaining portion of the values is greater than this threshold. 
     For the measurement of the distance a1 to the bottom of the vapor capillary  88 , a quantile of about 95% has been found suitable. This means that the “correct” distance value has the property wherein 95% of all measured distance values are smaller, while only 5% of the measured distance values are larger. In  FIG. 17 , the 95% quantile z q  is marked; wherein those bars of the histogram corresponding to the smaller distance values are also highlighted by hatching. The value of the quantile may be determined from a given histogram using algorithms known per se. 
     With a quantile filter, a realistic value for the distance a2 to the surface  91  of the workpiece  24  may also be determined. z is to be considered here. For example, a 5% quantile filtering applied to the left half of the histogram in  FIG. 17  is used. In this procedure, the risk that the determined value does not correspond to the actual value, however, is relatively large. The reason for this is that the number of distance values relating to the distance a2 is often much smaller than the number of distance values detected for the distance a1. Then, when the number of distance values for the distance a2 varies because the reflectance of the surface  91  fluctuates (due to, for example, surface contamination of the surface  91 ), these variations immediately noticeably affect the value of the quantile. The same applies to variations in the number of distance values caused by interferences or reflections from the vapor capillary  88 . Because of the comparatively small number of values for the distance a2, variations of the interference level also have a comparatively strong effect on the value of the quantile. 
     In many cases, it is therefore better to use a distance value in the interval with the greatest frequency as the actual value for the distance a2, starting from the histogram shown in  FIG. 17 . The maximum  142  falls in this interval in the histogram of  FIG. 17 . An average obtained from all distance values in this interval may then be determined, for example, as a distance value. Furthermore, it is also possible to include the distance values from a predetermined number of neighboring intervals in the averaging. 
     ii) Other Filters 
     Instead of the quantile filter, other filters may also be used to derive the distances a1 and a2 from the measured distance values. In particular, it is possible to derive a distribution function with certain distribution parameters from the histogram according to  FIG. 17  with the aid of a curve fit. In  FIG. 17 , such a distribution function is shown with a dashed line and designated  144 . Values for the actual distances a1, a2 may then be derived from the parameters of the distribution function. The distances a1, a2 need not necessarily correspond to the maxima of the distribution functions. For example, beta distribution (with 0, β&lt;1) and the Johnson-SU distribution are suitable as distribution functions. 
     Often it will be useful not to fit the entire histogram, but only the two halves of the histogram, wherein each contains one of the maxima  140 ,  142  with a distribution function. The distribution functions include, in particular, the following functions: Gaussian distribution, Poisson distribution, gamma distribution, Chi-square distribution, Lognormal distribution and Pearson distribution. 
     It has been found that it may be better not to use the same filter for the distances a1, a2, but different filters. Thus, experiments have shown in many cases that a quantile filter provides particularly good results for the distance a1 to the bottom of the vapor capillary  88 . For the distance a1 to the surface  91  of the workpiece  24 , however, it may be more appropriate to derive it from the parameters of a fitted distribution function. 
     4. Result 
       FIG. 18 a    shows a diagram in which, similarly to  FIG. 7 , the values for the distance z obtained by the Fourier transform from the interference spectra are plotted over the time t. It was assumed here that disturbances in the spectra were not removed, as explained in Sections  3  and  4 . An artifact  146  may therefore be seen in the diagram, which lies on an approximately horizontal line and is in the vicinity of the distance values, which are evaluated for the determination of the distance a1. 
     In addition, several artifacts  148   a ,  148   b , and  148   c  are in the vicinity of the distance values that are evaluated for the determination of the distance a2. These artifacts  148   a ,  148   b  and  148   c  are also approximately on a horizontal line but are temporally interrupted. 
     The solid lines  150  and  152  show the distances a1 and a2, respectively, which correspond to the 95% and 5% quantiles, respectively, as explained above in section  4   a ). The artifact  146  appears to increase the values for the distance a1. The intermittent artifacts  148   a ,  148   b , and  148   c  cause jumps in the values for the distance a2, although the surface  91  is flat except for a mid-point kink. 
       FIG. 18 b    shows a diagram corresponding to  FIG. 18 a   , except that the undisturbed interference spectra, not the disturbed spectra, were evaluated here. The artifacts  146   15  and  148   a ,  148   b ,  148   c  are no longer present. Accordingly, the distances a1 are slightly lower, which is closer to reality, and the distances a2 no longer show any apparent jumps, but correctly reflect the shape of the surface  91 .