Abstract:
This disclosure relates to verifying a seam quality during a laser welding process. In certain aspects, a method includes detecting, in a spatially resolved manner, a first amount of radiation emerging from a workpiece in a first wavelength range, determining a first geometric parameter of a seam characteristic based on the first amount of radiation detected in the first wavelength range, detecting, in a spatially resolved manner, a second amount of radiation emerging from the workpiece in a second wavelength range, the second wavelength range being different than the first wavelength range, determining a second geometric parameter of the seam characteristic based on the second amount of radiation detected in the second wavelength range, comparing the first and second geometric parameters to respective reference values or to respective tolerance intervals to provide respective comparison results, and logically combining the respective comparison results to verify the seam quality.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority under 35 U.S.C. §119 to European Patent Application No. EP 11 180 542.0, filed on Sep. 8, 2011. The contents of this priority application are hereby incorporated by reference in their entirety. 
       TECHNICAL FIELD 
       [0002]    The present disclosure relates to a method of verifying seam quality during a laser welding process. 
       BACKGROUND 
       [0003]    When non-optimal welding parameters are set for laser radiation welding, welding defects may occur, which can lead to seam irregularities. Such irregularities may include incomplete penetration, humping (e.g., irregularities in the curvature of a weld bead), and holes. Observation of a welding process may enable the detection of various welding and seam defects. However, reliable detection of defects may be disrupted by process emissions, such as process plasma or thermal radiation from a metal vapor plume or result from a surface finish on the weld bead. 
       SUMMARY 
       [0004]    The present disclosure relates to reducing a susceptibility of welding process monitoring to errors. In one aspect of the invention, a method of verifying a seam quality during a laser welding process includes detecting, in a spatially resolved manner, a first amount of radiation emerging from a workpiece during the laser welding process in a first wavelength range, determining a first geometric parameter of a seam characteristic based on the first amount of radiation detected in the first wavelength range, detecting, in a spatially resolved manner, a second amount of radiation emerging from the workpiece during the welding process in a second wavelength range, the second wavelength range being different from the first wavelength range, determining a second geometric parameter of the seam characteristic based on the second amount of radiation detected in the second wavelength range, comparing the first and second geometric parameters to respective reference values or to respective tolerance intervals to provide respective comparison results, and logically combining the respective comparison results to verify the seam quality. Radiation emerging from the workpiece may be radiation that is generated during the welding process or measurement light that is reflected from the workpiece. 
         [0005]    In some implementations, spatially resolved observation of the welding process may occur in two different spectral ranges, and combined evaluation of resultant measured values may make the detection of seam irregularities more reliable. Combination of the measurements can occur after comparing the parameters to respective reference values or reference intervals. Since the result of such a comparison is generally a binary value, combination of the results may occur in the form of logical combining. 
         [0006]    Simultaneous capture of different spectral ranges enables the determination of two different geometric parameters, which may be visible in one of the two spectral ranges used and refer to the same seam characteristic. For example, when butt-welding structural steel, full penetration may be detected by determining seam concavity as the first geometric parameter and weld pool length as the second geometric parameter. 
         [0007]    A seam characteristic (e.g., a seam width) may also be determined as the first parameter by direct measurement, for example, by a light section, in the visible spectral range. Weld pool width, which is determined in the infrared wavelength range and is likewise a measure of seam width, may serve as the second seam width parameter. 
         [0008]    The seam characteristic may be selected from the group including full penetration, seam width, humping, and a degree of asymmetry during welding of the workpieces. 
         [0009]    To test full penetration, the presence or absence of a full penetration hole may, for example, be determined both in the visible and infrared wavelength ranges. In this case, the two geometric parameters are the same, and a redundant determination is carried out in the two different wavelength ranges. However, if the full penetration hole cannot be detected in one of the wavelength ranges (e.g., in the visible wavelength range), it may alternatively be possible to determine seam concavity or weld pool length as a parameter for full penetration. 
         [0010]    As discussed above, the seam width may be determined in both wavelength ranges. Humping (i.e., variation in the curvature of the seam) may also be determined by detection of a light section in the visible range and by detection of variation of the heat trail width of the cooling seam in the infrared wavelength range. 
         [0011]    Asymmetry during welding of the workpieces may arise, for example, if the workpieces exhibit a height discrepancy or a difference in thickness at the joint on welding, or if the two workpieces are made of materials with very different melting points, heat conduction properties, and/or heat capacities. To characterize a degree of asymmetry, the distance between the focal spot and the joint, which may be determined in the visible wavelength range, may serve as the first parameter, and the lateral position of the weld pool and/or the lateral position of the heat trail of the cooling weld seam in the infrared wavelength range may serve as the second parameter. 
         [0012]    In some implementations, accuracy may be increased by accounting for the first geometric parameter when determining the second geometric parameter (or vice versa). For example, the influence of the first geometric parameter on the spatially resolved detected radiation in the second wavelength range or the spatially resolved measured radiant intensity may be taken into account. In some examples, the first geometric parameter may be a measure of the curvature of the weld bead, for example, a distance between the lowest or highest point on the top of the seam (perpendicular to the feed direction) and the workpiece surface, which may, for example, be determined with a light section. 
         [0013]    Knowledge about the shape of the weld bead or the waviness thereof may be used to increase precision in determining parameters that are dependent on the heat trail (e.g., in the infrared spectral range) since the curvature of the weld bead influences the spectral beam density generated from the heat trail of the seam. In such a case, the spectral beam density depends both on temperature and on emissivity, wherein the emissivity is dependent inter alia on the angle of the seam surface relative to the detector beam path. Emissivity therefore depends on the shape or curvature of the seam surface or the weld bead. The curvature of the weld bead may therefore be used, as a first parameter, to compensate for the influence of the varying emissivity through the curved surface of the seam on the detected beam density of the thermal image and, for example, to enable determination of the width of the heat trail with greater accuracy. 
         [0014]    In some implementations, seam quality is assessed as unsatisfactory if the parameter does not correspond to the reference value in at least one wavelength range or lies outside of the tolerance interval. Comparison of the parameter to the reference value or to the tolerance interval typically leads to a binary decision about the respective seam characteristic (e.g., full penetration present or not present, full penetration hole detected or not detected, seam width “OK” or unsatisfactory, humping present or not present, asymmetry present during welding or not present during welding, or weld point in correct place or not in correct place). These binary results may be logically linked for assessment of the respective seam characteristic. Thus, in some implementations, the seam quality may be assessed as “OK” if the result matches in both cases and indicates good seam quality (e.g., where the results are full penetration hole present or humping not present). 
         [0015]    In certain embodiments, the method includes improving the seam quality by modifying parameters of the laser welding process. If the seam quality is assessed as unsatisfactory, an attempt may be made to modify the parameters of the welding process such that the seam quality again falls within the tolerance interval or corresponds to the reference value. Accordingly, welding parameters such as welding speed, laser radiation intensity, laser machining head position, and clamping forces of the workpieces to be welded, may be modified in a manner that improves the seam quality. For example, a closed-loop control may be provided as a result of the seam quality assessment. In some examples, if asymmetry of the fusion of the workpieces with regard to the joint or deviation of the focal spot from the desired weld position relative to the joint is determined as the seam characteristic, a control loop, which is typically present anyway and which serves for determining the lateral position of the focal spot relative to the joint, may be used for correction. 
         [0016]    In some embodiments, the first wavelength range is the visible (VIS) spectral range, and the second wavelength range is the infrared (IR) spectral range. The VIS spectral range includes wavelengths of between about 380 nm and about 800 nm. The IR spectral range includes wavelengths of over about 800 nm to about 1 mm. The near infrared (NIR) range is used for some applications and includes wavelengths in the IR range that are about equal to or less than 2 μm. 
         [0017]    For measurement in the two spectral ranges, two cameras with sensitivities in the VIS or NIR ranges may be used. Alternatively, a single camera with a dual bandpass filter in the beam path may be used to transmit visible radiation in one range and NIR radiation in another range, which are captured in different zones on the detector surface of the camera. The NIR range may additionally be subdivided into two or more sub-ranges, for example, in order to allow absolute temperature measurement on the basis of quotient pyrometry. Bandpass filters may be used in the camera beam path that transmit radiation or process light in the appropriate spectral range and exclude process light outside of the appropriate spectral range. 
         [0018]    In certain embodiments, the first parameter is selected from the group including an extent of a full penetration hole, a distance between a laser focal spot and a joint, a curvature of the weld bead, and a seam width. In some examples, these parameters are particularly easy to measure in the VIS wavelength range. The location of the joint may, for example, be determined with the assistance of reflected light and/or light section illumination in order to determine the distance to the laser focal spot, whose position may typically likewise be detected in the VIS range. The location and shape of the imaged seam, for example, the curvature or the geometry of the weld bead, may also be detected with the assistance of light section illumination. 
         [0019]    In some embodiments, the second parameter is selected from the group including an extent of a full penetration hole, a weld pool length, a weld pool width, a width of the heat trail, and a distance between a weld pool or a heat trail and a joint. For example, the weld pool and the heat trail adjoining this in the cooling melt of the material of the weld seam may readily be observed in the NIR wavelength range. The position of the laser focal spot may also be detected in this wavelength range. 
         [0020]    In certain embodiments, the radiation detected in the first or second wavelength range may be used to determine at least one additional geometric parameter that is used to verify a plausibility of one or both of the first and second parameters. In some examples, the length of the weld pool may be determined as the additional parameter. The additional parameter may serve in the detection of a faulty measurement, for example, when determining the convexity or concavity of the weld seam using a light section. Such faulty measurements may occur if the length of the weld pool is so great that it extends into the surface zone in which the convexity or the concavity of the weld seam is to be determined using the light section. 
         [0021]    In some implementations, a combination of evaluating measured values or parameters from both spectral ranges enables a more robust detection of weld seam irregularities. Combined evaluation of the measurement results may take place in a “frame grabber” of a camera used as detector, in an image processing computer (e.g., in a laser machining machine controlling the welding process), or in an external control unit. 
         [0022]    Other aspects, features, and advantages will be apparent from the description, the drawings, and the claims. The above-stated features and those stated below may likewise each be used on their own or in any desired combinations. The embodiments illustrated and described should not be understood to be a comprehensive list, but rather, are used as examples to describe the embodiments. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0023]      FIG. 1  is a schematic representation of a laser machining head with two cameras for detecting radiation in two different wavelength ranges. 
           [0024]      FIG. 2  is a schematic representation of a plan view of various measurement zones that are used to determine parameters for seam characteristics during a laser welding process. 
           [0025]      FIG. 3  is a schematic representation of a workpiece with three differently curved weld seams. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]      FIG. 1  shows a laser machining head  1  with a focusing lens  2 , which focuses a laser beam  3  supplied to the laser machining head  1  onto a focal area on a workpiece surface  4   a.  The laser beam  3  is focused on a region of a joint site between two workpieces  4  in order to perform laser welding at the site. To monitor the welding process, a sensor device  5  with a CMOS camera  5   a  and an InGaAs camera  5   b  is provided on the laser machining head  1 . 
         [0027]    A beam path  6  of the CMOS camera  5   a  is reflected onto the workpiece surface  4   a  by a partially transmissive deflecting mirror  7 . In this case, not only the focal area of the laser beam  3 , but also a relatively large portion of the workpiece  4  in the area surrounding the laser beam  3  is imaged onto a detector surface  8  of the CMOS camera  5   a.  Like the beam path  6  of the CMOS camera  5   a,  a beam path  9  of the InGaAs camera  5   b  is also reflected onto the workpiece surface  4   a  by a deflecting mirror  10  and images the portion of the workpiece  4  in the area surrounding the laser beam onto a corresponding detector surface  11 . On the laser machining head  1 , there are additionally provided two line projectors  12   a,    12   b  associated with the sensor unit  5 , which project two laser light lines  13   a,    13   b  into the beam paths  6 ,  9  monitored by the CMOS camera  5   a  and the InGaAs camera  5   b.  The laser light lines  13   a,    13   b  are hereinafter also called light sections  13   a,    13   b.    
         [0028]    As a function of the measurement data captured by the cameras  5   a,    5   b,  the laser machining head  1  may be displaced along a first adjusting axis (e.g., a Y axis of an XYZ system of coordinates, where the X direction is the feed direction) implemented as a linear axis, so as to bring the laser beam  3  to its optimum (e.g., lateral) welding position. A second adjusting axis (e.g., a Z axis of the XYZ system of coordinates) may be used additionally to vary the distance between the laser machining head  1  and the workpiece surface  4   a.  Accordingly, a beam guide means  14  of the laser machining head  1  is displaceable in the respective axial direction by means of linear drives (not shown). 
         [0029]    Still referring to  FIG. 1 , the CMOS camera  5   a  serves to capture (e.g., to detect) radiation in a spatially resolved manner in a first wavelength range W 1 , for example, the visible (VIS) range (e.g., to capture wavelengths of between about 380 nm and about 800 nm). The InGaAs camera  5   b  serves to capture (e.g., to detect) radiation in a spatially resolved manner in a second wavelength range W 2 , for example, the near infrared (NIR) range (e.g., to capture wavelengths of between about 800 nm and about 2 μm). The NIR wavelength range W 2  may additionally be subdivided into two or more sub-ranges in order to allow, for example, absolute temperature measurement on the basis of quotient pyrometry. In the respective beam paths  6 ,  9  of the cameras  5   a,    5   b,  respective bandpass filters (not shown) may be used to transmit process light in the appropriate spectral range and, to the extent possible, to reject process light outside of the spectral ranges W 1 -W 2 . 
         [0030]    In some embodiments, the arrangement of the cameras  5   a,    5   b  relative to the beam path of the laser beam  3  may be different from that shown in  FIG. 1 . For example, just one deflecting mirror may be arranged in the beam path of the laser beam  3 , and assignment of the respective beam paths  6 ,  9  to the cameras  5   a,    5   b  may occur via a beam splitter. In some embodiments, a single camera (e.g., a camera with a dual bandpass filter in the beam path) may be used, which in a first zone, transmits radiation in the VIS wavelength range W 1  and in a second zone transmits radiation in the NIR wavelength range W 2 , which impinges on different areas of the detector surface of the camera. Furthermore, non-coaxial observation is possible, in which the beam paths  6 ,  9  of the cameras  5   a,    5   b  are not congruent with the beam path of the laser beam  3 . 
         [0031]      FIG. 2  illustrates a plan view of various measurement zones at the detector surface  8  that are used to determine parameters for seam characteristics during a laser welding process. In some implementations, a quality of the seam may be improved by modifying such parameters of the laser welding process. Three measurement zones  15   a - 15   c  are arranged at the detector surface  8  of the CMOS camera  5   a  along a welding direction (e.g., an X direction) indicated by an arrow in  FIG. 2 . During the welding process, the first measurement zone  15   a  (e.g., a pre-measurement window) captures a portion of the workpieces  4  while being moved forward to a focal spot  17 , which is produced by the laser beam  3 . In the first measurement zone  15   a,  a position or a shape of a joint  18  between the workpieces  4  is captured using the first laser light line  13   a  (i.e., the light section  13   a  shown in  FIG. 1 ). Alternatively, the joint  18  may also be extensively illuminated by the light section  13   a  and captured in the reflected light. 
         [0032]    In the second measurement zone  15   b  (e.g., a post-measurement window), radiation is detected after welding. For example, the second laser light line  13   b  allows information to be obtained about the geometry (e.g., a convexity or a concavity) of a weld seam  19 . The third measurement zone  15   c  (e.g., an in-measurement window) allows for observation of the direct welding zone with the focal spot  17 , which corresponds to the area at which the laser beam  3  impinges on the workpiece surface  4   a.    
         [0033]    At the sensor face  11  of the InGaAs camera  5   b,  a measurement zone  16  is evaluated, which captures thermal radiation in the region of the focal spot  17  of the adjacent weld pool  20  and a heat trail  21  (illustrated schematically in  FIG. 2 ) of a cooling melt that adjoins the weld pool  20 . The respective measurement zones  15   a - c ,  16  correspond to the zones in which the intensity of the light irradiated onto the respective sensor faces  8 ,  11  are evaluated by an evaluation device (not shown). Combined evaluation of the measurement results may take place in a “frame grabber” of the camera  5   a,    5   b  used as a detector in an image processing computer (e.g., in a laser machining apparatus that controls the welding process, or alternatively, in an external control unit). Although the measurement zones  15   a - c,    16  have rectangular shapes in the example embodiment of  FIG. 2 , in other embodiments, the measurement zones  15   a - c,    16  may take on other shapes or geometries. In a general embodiment, the number of measurement zones may also vary. 
         [0034]    Using the cameras  5   a,    5   b,  a first or second geometric parameter may be determined in the first or second wavelength range W 1 , W 2 , respectively, that describes the same seam characteristic. The geometric parameter may take the form, for example, of a spatial extent or a geometry of a full penetration hole that lies within the region of the focal spot  17  in the example shown in  FIG. 2 . When using a solid-state laser to generate the laser beam  3 , the full penetration hole is visible in both VIS and NIR wavelength ranges W 1 , W 2 , respectively. In such a case, the first and second parameters describe the same geometric variable (e.g., the extent, and in particular, the size of the full penetration hole). By simultaneous detection of the full penetration hole in both spectral ranges W 1 , W 2 , plausibility testing can be performed, and the detection rate can be increased. 
         [0035]    The full penetration hole may be captured as a spatially delimited, reduced illuminance on the cameras  5   a,    5   b  within the respective measurement zone  15   c,    16  in the area surrounding the focal spot  17 . Additional criteria, such as a minimum size of a detected hole, a contrast, or a location of the detected hole, may be taken into consideration as reference values for detection. In some implementations, comparison of the spatially delimited, reduced illuminance to the respective reference value leads either to detection or to no detection of the full penetration hole and thus yields a binary result (e.g., a full penetration hole present or a full penetration hole not present). Accordingly, the two results of the comparison in the two spectral ranges W 1 , W 2  may be logically combined in order to assess seam quality. In some implementations, the welding process or seam quality may be assessed as satisfactory or as “OK” if the full penetration hole is detected in both spectral ranges W 1 , W 2  (i.e., when the full penetration hole is present). 
         [0036]    Still referring to  FIG. 2 , when welding with a CO 2  laser, the full penetration hole may only be visible in the NIR spectral range W 2  since the full penetration hole is concealed in the VIS spectral range W 1  by plasma in a keyhole. In the NIR spectral range W 2 , the full penetration hole may additionally be directly detected as a full penetration parameter. In some implementations, when butt-welding structural steel, the concavity of the weld seam  19  formed may additionally be determined as a parameter in the VIS spectral range W 1  using the second light section  13   b.  In such a case, a distance L 1  between the workpiece surface  4   a  and the lowest or highest point of the seam serves as a measure of convexity or concavity, where both variables or the distance L 1  may be simply determined using the second light section  13   b,  as illustrated in  FIG. 2 . The distance L 1  measured in this way is compared with a reference distance L 1 REF determined, for example, during test welding. A weld pool length L 2  is an additional parameter that can be detected in the NIR spectral range W 2 . If the weld pool length L 2  is smaller than a predetermined reference value, (e.g., L 2 REF, determined by test welding), this may be indicative of full penetration. Thus, if the distance L 1  corresponds to the reference distance L 1 REF, the length L 2  is smaller than the reference value L 2 REF, and the presence of the full penetration hole is detected, the seam characteristic “full penetration” may be assessed as “OK.” 
         [0037]    When measuring a width of the seam  19 , plausibility testing may be performed via measurement in both spectral ranges W 1 , W 2 . For example, a seam width (B 1 ) may be determined as a first parameter in the VIS spectral range W 1  via the second light section  13   b.  The result of comparing the width B 1  with a reference value BREF (shown in  FIG. 3 ) is verified by measuring a second parameter, a weld pool width B 2  (shown in  FIG. 2 ). The width B 2  of the weld pool  20  in the NIR spectral range W 2  typically corresponds to the seam width B 1  and is likewise compared with the reference variable BREF, which corresponds to a desired (e.g., ideal) weld seam width. 
         [0038]    In some implementations, measurement of the seam width B 1  via the second light section  13   b  and measurement of the weld pool width B 2  in the NIR wavelength range W 2  may occur at different locations in the welding direction (e.g., the X direction as shown  FIG. 2 ). Accounting for a weld speed or a time offset, the widths B 1 , B 2  determined in each case may be assigned to the same position along the weld seam  19  in order to perform the comparison between the parameters B 1 , B 2  at the same location. 
         [0039]      FIG. 3  illustrates a workpiece with three different weld seams that have different curvatures. In some implementations, the width B 1  of the weld seam  19  (shown in  FIG. 2 ) does not match the reference width BREF exactly. However, differences between the width B 1  of the weld seam  19  and the reference width BREF that are within a tolerance interval BREF+/−BT may be assessed as tolerable for seam quality, and consequently, seam quality may be assessed as satisfactory. For example, the seam quality may be assessed as satisfactory if B 1  lies substantially between BREF−BT and BREF+BT or if B 2  lies substantially between BREF−BT and BREF+BT. Accordingly, the seam quality may be assessed as unsatisfactory if B 1  does not lie substantially between BREF−BT and BREF+BT or if B 2  does not lie substantially between BREF−BT and BREF+BT. 
         [0040]    Still referring to  FIG. 3 , in some implementations, “humping” (i.e., a seam defect that describes a variation in the seam convexity or concavity) of the seam  19  may be detected as an additional seam characteristic. For example, a first weld bead  19   a  exhibits a seam convexity, a second weld bead  19   b  exhibits a seam concavity, and a third weld bead  19   c  provides a transition between the seam convexity and the seam concavity. 
         [0041]    Referring again to  FIG. 2 , a regularly recurring variation in seam curvature (e.g., humping) may be detected by determining the distance L 1  between the workpiece surface  4   a  and the lowest or highest point of the seam  19  in the VIS wavelength range W 1  using the second light section  13   b.  For example, variation in the length L 1  (e.g., as measured as a variance or a difference between the minimum and maximum) over a predetermined period of time can serve as the first parameter. The second parameter can be measured as a variation in a width B 3  of the heat trail  21  in the NIR wavelength range W 2  over the same period of time since the width B 3  of the heat trail  21  is dependent on the seam convexity or concavity. In some examples, the width B 3  of the heat trail  21  is dependent on the convexity or concavity because more molten material is present at seam convexities, such that the detected heat trail becomes wider. If the result of a comparison of the two parameters with a respective reference value for the variation is that the variation lies within a tolerance range, it may be concluded that humping is not present, and thus, seam quality may be assessed as acceptable. 
         [0042]    Asymmetry during fusion of the workpieces  4  provides an additional seam characteristic. Such asymmetry may result due to a height discrepancy or a difference in thicknesses of the workpieces  4  at the joint  18  or, in the case of workpieces of different materials (e.g., steel and cast iron), due to the workpieces exhibiting different melting points, different heat conduction properties, or different heat capacities. For weld asymmetry, the distance between the lateral position of the focal spot P and the joint  18  may be measured as the first parameter in the VIS spectral range W 1 . In parallel, in the NIR wavelength range W 2 , the distance may be measured between the lateral position of the weld pool  20  and/or the heat trail  21  and the joint  18  or the position P of the focal spot  17 . 
         [0043]    In some implementations, the seam quality is assessed as “OK” if the position P of the focal spot relative to the joint  18  is the first parameter, and the center of gravity of the weld pool  20  or a middle M of the weld pool  20  at its widest point of the area center of gravity of the heat trail  21  in a direction transverse to the joint  18  is the second parameter and exhibits a reference distance previously determined, for example, by test welds, or the distance is smaller than said established reference distance (e.g., the distance in  FIG. 2  is equal to zero). 
         [0044]    If upon comparison of at least one of the two parameters with this reference distance, too great a deviation arises, a seam location control system control loop may be used to compensate for the asymmetry during fusion of the parts to be joined. Furthermore, the control loop may be used to increase the accuracy of the seam location control system, for example, by correcting the position of the laser machining head  1  in a direction that is transverse to the joint gap  18  (e. g., in the Y direction as shown in  FIG. 2 ). This is particularly favorable if, upon joining a pair of materials, a defined mixing ratio of the materials is established in the weld seam  19 . In some implementations, even if an unsatisfactory seam quality is detected, appropriate adjustment of laser welding process parameters may be carried out using the above-described seam characteristics. Example welding parameters that may be modified include laser intensity, welding speed, and other parameters. 
         [0045]    Referring to  FIGS. 2 and 3 , in the case of flat weld beads, (e.g., seams lacking curvature), measurement of the weld bead width B 1  and the lateral weld bead position with the second light section  13   b  in the VIS spectral range W 1  may be problematic, since light section measurements may require a contour modification perpendicular to the workpiece surface. In contrast, local convexity or concavity of the weld beads  19   a - c  may be measured with the light section as the first parameter. In the NIR wavelength range W 2 , the weld pool width B 2  may be determined as the second parameter and thus may effectively correspond to the weld bead width, but not to the convexity or concavity of the weld bead  19   a - c  (see, for example,  FIG. 3 ). Using combined measurements in both wavelength ranges W 1 , W 2 , both the weld bead width and local seam convexities or concavities may be measured for flat weld seams. Seam quality, and thus, the welding process, are assessed as “OK” if the two parameters L 1 , B 2  display values established by test welds or lie within tolerance ranges that have been previously established. 
         [0046]    To evaluate the above-described measurement, the weld pool length L 2  may be used as an additional parameter. For example, the weld pool length L 2  is checked to determine whether it exceeds a predetermined reference length, which need not necessarily match the reference length L 2 REF for assessment of full penetration. If L 2  does exceed the predetermined reference length, the weld pool  20  may project at its rear end into the region of the light section  13   b,  such that the measurement of seam convexity or concavity in the VIS wavelength range W 1  is generally incorrect. If the result of the comparison with the reference length is that the weld pool  20  is too long, the corresponding measurement of the length L 1  by means of the second light section  13   b  in the VIS wavelength range W 1  may not be used to determine the convexity or concavity of the seam. In some implementations, the weld pool length L 2  thus serves as a confirmation of the plausibility of the determination of the length L 1  as a measure of the concavity or convexity of the weld bead  19   a - c.    
         [0047]    In some implementations, when determining one of the two parameters, it is beneficial to account for the other parameter to increase precision. For example, the spectral radiance, which is based on the heat trail  21  of the welding process in the NIR wavelength range W 2 , is influenced by the (absolute) temperature and the emissivity at the surface of the weld bead  19   a - c . Emissivity is, however, dependent on the angle at which the radiation  9  (see  FIG. 1 ) to be detected is emitted from the weld bead  19   a - c  (i.e., emissivity is influenced by the shape of the weld bead). 
         [0048]    As discussed above, the shape of the weld bead  19   a - c  may be determined using light section measurement in the VIS spectral range W 1 . In such a case, the length L 1  between the workpiece surface  4   a  and the lowest or highest point of the weld bead  19   a - 19   c  may serve as a measure of the convexity or concavity of the weld bead  19   a - 19   c.  The parameter, determined in this way, may be used in the evaluation of the heat trail  21  of the seam  19  in the NIR spectral range W 2  in order to computationally compensate for the influence of the varying emissivity of the curved surface of the seam  19  on the detected beam density. In this way, accuracy when determining the second parameter in the form of the seam width B 3  in the NIR wavelength range W 2  may be improved by determining the convexity or concavity of the weld bead  19   a - c.    
         [0049]    As discussed above, assessment of seam quality may be further validated by determining two parameters for the same seam characteristic so that process monitoring is more reliable. In some implementations, more than two parameters may be used to assess the same seam characteristic, where the seam characteristic is assessed as “OK” when the more than two parameters lie within the respective tolerance range or match the reference value. In some implementations, multiple seam characteristics may be monitored or verified in parallel. In such cases, the welding process may be assessed overall as “OK” if each individual seam characteristic is assessed as “OK”. 
         [0050]    A number of embodiments have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. Accordingly, other embodiments are within the scope of the following claims.