Magnification offset correction procedure

Abstract

A method for calibrating a position of a laser beam in an apparatus comprising at least one optical unit for directing the laser beam, the at least one optical unit comprising a plurality of optical elements, the method comprising the steps of: setting a first optical configuration for the plurality of optical elements of the at least one optical unit, thereby directing the laser beam onto a measurement surface with a first focal spot size, measuring a first position in the measurement surface of the laser beam generated by the first optical configuration, setting a second optical configuration for the plurality of optical elements of the at least one optical unit, thereby directing the laser beam onto the measurement surface with a second focal spot size different from the first focal spot size, measuring a second position in the measurement surface of the laser beam generated by the second optical configuration, and determining at least one correction value based on the measured first position and the measured second position.

Description

[Technical Field]

[0001] The present invention relates to a method for calibrating the position of a laser beam in an apparatus comprising at least one optical unit for generating a laser beam, the at least one optical unit comprising a plurality of optical elements, the apparatus being for powder bed fusion bonding, such as, but not limited to, selective laser sintering and / or selective laser melting. [Background technology]

[0002] Powder bed fusion is an additive process that can process powdered raw materials, particularly metal and / or ceramic raw materials, into three-dimensional workpieces with complex shapes. To achieve this, a layer of raw powder is applied to a carrier and then irradiated with radiation (e.g., laser or particle radiation) in a site-selective manner according to the desired shape of the workpiece to be produced. The radiation that penetrates the powder layer causes heating, resulting in the raw powder particles melting or sintering. Further layers of raw powder are then applied successively to the previously irradiated carrier layer until the workpiece has the desired shape and size. Powder bed fusion may be employed to manufacture prototypes, tools, replacement parts, high-value parts, or medical prostheses, such as dental or orthopedic prostheses, based on CAD data. Examples of powder bed fusion techniques include selective laser melting and selective laser sintering.

[0003] Apparatuses for manufacturing one or more workpieces according to the above techniques are known. For example, EP 2961549 A1 and EP 2878402 A1 each describe an apparatus for manufacturing three-dimensional workpieces according to the selective laser melting technique. The general principles described in the above patent documents may also be applied to the techniques of the present disclosure.

[0004] When producing workpieces by powder bed fusion techniques involving at least one laser beam, it may be desirable to use a particular optical unit of the apparatus to generate laser spots with different laser spot sizes depending on the intended use. For example, a larger laser spot size may rapidly irradiate and solidify larger areas of the workpiece. In contrast, a smaller laser spot size may result in finer solidification of important parts of the workpiece, particularly the workpiece's outline (also called the shell). For this reason, it is known to irradiate the shell of a workpiece with a smaller laser spot size than the core. The core is typically irradiated, for example, with a so-called hatch pattern including parallel scanning vectors. The above method of solidifying the shell and core of a workpiece with different laser spot sizes is also called the shell-core method.

[0005] It is also known to simply defocus the laser beam (i.e., move the focal spot along the z-direction perpendicular to the powder bed in the xy-plane, so that the focal position no longer corresponds to the z-position of the powder bed) for the above purpose of providing different laser spot sizes, which also leads to a broadening of the spot size compared to the spot size at a precisely focused spot position (corresponding to the beam waist).

[0006] However, this disclosure is directed to actual differences in focal spot size at the focal position of the laser spot. As explained in more detail below, this focal spot size can be adjusted by employing a zoom optical system that involves moving at least two optical elements (e.g., at least two lenses) of an optical unit. Thus, when this disclosure refers to "focal spot size," it means the spot size of the laser light at its focal position (also called the beam waist).

[0007] The projected beam spot size at the projection plane (e.g., measurement plane) may depend on the focal spot size and the defocus of the laser light (i.e., the focal position along the z-direction with respect to the projection plane). If the focal spot size is changed and the defocus is changed at the same time, the projected beam spot size in the projection plane may even end up being the same size. Therefore, the projected beam spot size does not provide information about the focal spot size.

[0008] When laser spots with different focal spot sizes are generated, different optical configurations of the optical elements can result in different lateral focal spot positions within the plane in which the laser spot is directed (e.g., the xy plane within or parallel to the powder bed).

[0009] However, this lateral offset (i.e., offset in the xy plane) can cause an offset between the portion irradiated by the first focal spot size and the portion irradiated by the second focal spot size. In particular, in the shell-core method discussed above, this can result in an offset between the shell and core of the resulting workpiece layer. If this offset is too large, the workpiece may become defective and unusable.

[0010] Those skilled in the art will appreciate that the above problems with varying lateral positions of the laser spot may also pose problems in laser processing equipment other than equipment for powder bed fusion. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] European Patent Application Publication No. 2961549 [Patent Document 2] European Patent Application Publication No. 2878402 Summary of the Invention

[0012] Therefore, the present invention aims to provide a method for calibrating the position of a laser beam in an apparatus comprising at least one optical unit for generating a laser beam, the at least one optical unit comprising a plurality of optical elements, the method avoiding or reducing at least one of the above-mentioned problems or related problems. In particular, it is desirable to avoid a lateral offset of the focal position of laser spots having different focal spot sizes generated by one and the same optical unit.

[0013] This object is addressed by a method according to claim 1 as well as by a computer program product according to claim 15.

[0014] According to a first aspect, there is provided a method for calibrating a position of a laser beam in an apparatus including at least one optical unit for directing laser beam, the at least one optical unit including a plurality of optical elements, the method including: configuring a first optical configuration for the plurality of optical elements of the at least one optical unit to thereby direct laser beam onto a measurement surface with a first focal spot size; measuring a first position in the measurement surface of the laser beam generated by the first optical configuration; configuring a second optical configuration for the plurality of optical elements of the at least one optical unit to thereby direct laser beam onto the measurement surface with a second focal spot size different from the first focal spot size; measuring a second position in the measurement surface of the laser beam generated by the second optical configuration; and determining at least one correction value based on the measured first position and the measured second position.

[0015] The apparatus may be for generating three-dimensional workpieces according to powder fusion techniques. More precisely, the apparatus may be for generating three-dimensional workpieces by selective laser sintering and / or selective laser melting. In this case, the method for calibrating the position of the laser beam may be followed by typical steps of powder bed fusion techniques, such as selective laser melting or selective laser sintering. In particular, the method may involve depositing a first layer of raw material powder on a carrier of the apparatus. The first layer (as well as subsequent layers) may have a predetermined layer thickness, which may be adjusted or fixed for each layer depending on the type of powder deposition technique used. The powder layer may be deposited by any suitable technique. Several methods and apparatuses for generating raw material powder layers are known in the art. After depositing the first raw material powder layer, predetermined areas of the powder are irradiated with laser light according to CAD data of the workpiece and / or support structure to be manufactured. In this way, the first layer of the generated workpiece may be irradiated and thereby solidified directly on the carrier or on a support structure bonded to the carrier. In the next step, a second layer of raw material powder is deposited and predetermined areas of that layer are irradiated and solidified. In this way, the workpiece is created layer by layer.

[0016] The optical unit may include multiple optical elements, such as at least one mirror, at least one lens, and at least one grating. The optical unit may include a laser source, or the laser source may be external. The laser radiation may be guided to the optical element via at least one fiber, or through air or vacuum. In this regard, strictly speaking, the optical unit does not necessarily "generate" laser light (in the sense of including a corresponding laser source), but rather "directs" laser light. Directing the laser light may include directing the laser light to a predetermined location on the measurement surface. For this purpose, the optical unit may include a scanner unit including a pair of movable (especially tiltable) mirrors, such as galvanometer mirrors. The mirrors of the scanner unit may be flat or have focusing properties (i.e., positive refractive power). Other technologies for scanning the laser light to a desired location may additionally or alternatively be employed, such as at least one acousto-optic deflector, at least one piezoelectrically driven mirror, etc.

[0017] The optical unit may further comprise an optical element for focusing the laser light to a desired location along a z-axis, the z-axis being defined according to a Cartesian coordinate system, with the measurement plane parallel to the xy-plane and the z-axis extending perpendicular to this xy-plane.

[0018] The optical unit may further comprise zoom optics, which allows to vary the focal spot size (i.e. the beam waist at the focal position). Such functionality (i.e. varying the focal position and varying the focal size) may be at least partly employed by one and the same optical component, such as one or more movable lenses (more precisely, lenses that are movable along the optical axis).

[0019] The expression "optical configuration" as used herein includes a predetermined position of an optical element within an optical unit. In other words, in a first optical configuration, at least one optical element within the optical unit has a different position than in a second optical configuration. Here, the position may be determined relative to a reference position within the optical unit. Here, the reference position may be fixed relative to the housing of the optical unit. In particular, a different position may refer to a different position of one or more optical elements along the optical axis.

[0020] In other words, at least one of the plurality of optical elements may be moved along the optical axis to move the optical unit from a first optical configuration to a second optical configuration. For example, at least two of the plurality of optical elements may be moved along the optical axis to move the optical unit from the first optical configuration to the second optical configuration. According to this disclosure, the optical axis is the optical axis of the optical system formed by the optical elements in the optical unit.

[0021] The apparatus may include a plurality of (e.g., two, four, eight, or twelve) optical units, each configured to direct laser light onto a measurement surface. In this case, each of the plurality of optical units may include one or more of the optical elements discussed above. In particular, each optical unit may include a scanner unit, focusing optics, and zoom optics.

[0022] When the optical unit is set to the first optical configuration, it generates a focal spot in the measurement plane. The measurement plane is parallel to the xy plane. The measurement plane may be identical to the top layer of raw material powder applied by the apparatus (the so-called build plane). In this case, the raw material powder may be illuminated for calibration. Alternatively, the measurement plane may be positioned at the build plane, i.e., the location where the top layer of raw material powder is placed during the construction of the three-dimensional workpiece. In this case, a sensor, foil, or plate may be positioned on a carrier of the apparatus, and the carrier is positioned along the z-axis so that the sensor, foil, or plate is positioned in the desired measurement plane. Alternatively, the measurement plane may be positioned in the same plane but next to the top layer of raw material powder (e.g., in the bottom region of the processing chamber of the apparatus). Furthermore, the measurement plane does not have to be parallel to the xy plane; for example, to perform calibration, the laser light may be deflected to a side region of the processing chamber by a movable mirror. In this case, the measurement plane may be parallel to the z-axis of the apparatus (e.g., the xz or yz plane).

[0023] In a preferred embodiment, in the first optical configuration, the focal spot is located in the measurement plane and has a first focal spot size. The first focal spot size may be determined by the focal spot diameter. Because the focal position of the laser light is located in the measurement plane, the first focal spot size may be determined by the beam waist of the laser light. The same applies to the focal spot size in the second optical configuration. The second focal spot is located in the measurement plane, and its size may be determined by the focal spot diameter. Because the focal position of the laser light is located in the measurement plane, the second focal spot size may be determined by the beam waist of the laser light.

[0024] The first and second positions may be measured by a sensor (e.g., a CCD sensor or a CMOS sensor) positioned on the measurement surface. Furthermore, the position may be measured by a caustic measurement device at the focal position of the laser light. Alternatively, the position may be measured by a camera. The camera may be positioned in the upper region of the processing chamber of the apparatus, and the camera's field of view may, for example, cover the entire measurement surface. The camera may measure the position when the laser light is directed at the measurement surface. Additionally or alternatively, the laser light may be used to burn an irradiation pattern (i.e., a mark) into a powder (meaning that the powder is melted to produce a solidified structure), a foil, or a plate (e.g., a photosensitive foil or anodized aluminum). In this case, the irradiation pattern may be detected by the camera to determine the position of the laser light. In particular, the irradiation pattern may be detected by the camera after the laser light has completed irradiation of the powder, foil, or plate. Furthermore, the laser light may burn an irradiation pattern (i.e., a mark) into the powder, foil, or plate, after which the structure, foil, or plate is removed from the apparatus and observed by an external device, e.g., a camera. In this case, the camera determines the first position and the second position. The camera used to measure the first and second positions may at least partially use the optical path of the laser light. In this case, the camera may be disposed within the optical unit.

[0025] It should be noted that this technique is not limited to measuring only one first position (for the first optical configuration) and only one second position (for the second optical configuration). Instead, multiple first positions and / or multiple second positions may be measured. In particular, if an illumination pattern is illuminated by laser light in the first optical configuration and / or the second optical configuration, multiple positions of this illumination pattern may be measured.

[0026] When the first and second positions are measured without changing the deflection of the scanning optics (e.g., the center of the optics), one offset is determined and the entire "field of view" is shifted for each magnification value. However, according to an improved method, measurements can be performed at multiple locations (within the scanning range of the optical unit). In this case, instead of determining only one offset value for one specific magnification value, multiple offset values ​​are determined and distributed over the scanning range (grid points). Between these grid points, offset values ​​can be interpolated (i.e., offset values ​​corresponding to positions between grid points can be interpolated based on the offset values ​​of the grid points). This technique can be considered as image plane correction for each magnification value. Interpolation can be performed not only between specific magnification values, but also for different deflections of the scanning optics for magnification values ​​(resulting in different nominal xy positions within the scanning range). To determine multiple deflections / positions for one specific magnification value, multiple concentric circles can be illuminated at different nominal positions in the scanning range (i.e., the centers of the circles have different nominal positions). The respective positions (and thus the offset values) may be detected, for example, using an on-axis or off-axis camera system.

[0027] A set of sensors may be arranged within the machine and may, for example, observe the entire building surface or only specific areas of the building surface. Alternatively, sensors may also be provided on or within the optical unit of the device and may partially use the optical path of the laser light. In this case, the observation area may also be movable via the scanner unit. The sensors used may in particular be optical sensors. The optical sensors may be sensitive to a wide wavelength spectrum or only to a specific spectrum, in particular the spectrum of laser light or infrared (IR) radiation (optionally using a filter that blocks the laser wavelength if it is within the detectable spectrum of the sensor).

[0028] The correction values ​​are determined based on the measured first position, in particular the first position data set, and the measured second position, in particular the second position data set. The at least one correction value may be indicative of a lateral shift (in the xy plane) of the laser light between the first and second optical configurations when the scanner unit of the optical unit remains in the same position. In other words, the at least one correction value may include an x-coordinate and a y-coordinate indicative of the lateral shift.

[0029] As described above, a plurality of first positions for the first optical configuration and a plurality of second positions for the second optical configuration may be determined. In this case, a correction value may be determined for each of the plurality of first positions. For locations that do not correspond to the plurality of first positions, a correction value may be interpolated or extrapolated based on the correction values ​​for the plurality of first positions.

[0030] The correction value may be used to eliminate lateral misalignment during use of the device, as described below.

[0031] The step of setting a first optical configuration for the plurality of optical elements of the at least one optical unit and / or the step of setting a second optical configuration for the plurality of optical elements of the at least one optical unit may include a step of focusing laser light onto the measurement surface.

[0032] Thus, the phrase "directing laser light onto the measurement surface with a first focal spot size" in the first aspect may be replaced with "focusing the laser light onto the measurement surface with a first focal spot size." Furthermore, the phrase "directing laser light onto the measurement surface with a second focal spot size" in the first aspect may be replaced with "focusing the laser light onto the measurement surface with a second focal spot size." Thus, at least one measurement of the laser light in the measurement surface may relate to the focal spot size in the measurement surface.

[0033] The method may further include storing a first position data set based on the measured first position and a second position data set based on the measured second position, where the determining step includes determining at least one correction value based on the first position data set and the second position data set.

[0034] The first position data set and the second position data set may be stored in the memory of the device. For example, the first position data set may indicate the x and y positions of the laser light within the measurement plane. For example, the first position data set may define a reference point (zero point) within the measurement plane. The x and y coordinates are determined from the reference point.

[0035] The method may further include applying at least one correction value during use of the apparatus so that a relationship between the position of the laser light in the first optical configuration and the position of the laser light in the second optical configuration is known.

[0036] For example, the control unit of the device may apply a correction value when the optical unit is in the second configuration, but not when the optical unit is in the first configuration. Similarly, the control unit of the device may apply a correction value when the optical unit is in the first configuration, but not when the optical unit is in the second configuration. Such an option may be used when the correction value indicates a lateral deviation of the laser light in the measurement plane between the first and second configurations. However, the correction value may also be calculated for the first and second optical configurations with respect to a reference point in the measurement plane. In this case, the correction value may be applied to both the first and second optical configurations. In either case, the correction value can be used to determine the position of the laser light in the second optical configuration compared to the first optical configuration, so that the laser light can be directed to a desired location in the x-y plane for each optical configuration.

[0037] The at least one correction value may be applied by a control unit of the apparatus during the generation of the three-dimensional workpiece. For example, the position data and / or steering data provided to the scanner mirrors may be adapted to take the at least one correction value into account. However, in addition to this, the build data of the workpiece may be modified to take the at least one correction value into account. In this case, the modification may be performed by an external device (e.g., a computer).

[0038] Furthermore, correction of lateral offset may be achieved by moving at least one optical element (e.g., along the optical axis and / or orthogonal to the optical axis), by rotating at least one optical element about one or more axes orthogonal to the optical axis, and / or by moving the optical unit with respect to the construction surface of the apparatus.

[0039] In addition to the first and second optical configurations, at least a third optical configuration may be considered, and the position of each may be measured. In this way, for example, at least four, at least six, at least eight, or at least ten optical configurations of the optical unit may be considered. Correction values ​​may be stored for each optical configuration. Each optical configuration may represent a magnification of the focal spot size. For example, the position of the laser spot may be measured for multiple integer magnification values ​​(such as 1, 2, 3, 4, 5, 6, 7, and 8) or non-integer magnification values.

[0040] Locations that do not correspond to the considered optical configuration (i.e., the positions measured for each focal spot size) may be interpolated or extrapolated as discussed below.

[0041] The method may further include determining at least one correction value for the third optical configuration by performing interpolation or extrapolation based on the at least one correction value.

[0042] Linear interpolation and / or linear extrapolation may be applied. More precisely, it may be assumed that there is a linear dependence between the two known correction values, between the focal spot size (e.g., magnification) and the lateral displacement in the x-y plane. Furthermore, higher orders of interpolation and / or extrapolation may be considered.

[0043] Interpolation and / or extrapolation may be applied based on changes in focal spot size (e.g., magnification) or, alternatively, based on changes in position (e.g., distance traveled) of an optical element.

[0044] Setting the second optical configuration for the optical elements of the at least one optical unit may comprise changing the position of at least two optical elements, in particular at least two lenses.

[0045] In other words, at least two optical elements may be moved to move the optical unit from the first configuration to the second configuration. In particular, at least two lenses may be moved along the optical axis. Each of the two lenses may have a positive refractive power. The at least two lenses that are moved may be in a telescope configuration. Furthermore, the optical unit may include three or more lenses, for example, four lenses. The four lenses may be arranged in a double telescope configuration. In this case, at least four lenses may be moved to move the optical unit from the first optical configuration to the second optical configuration, for example.

[0046] The method may further include illuminating the measurement surface according to a first illumination pattern while in the first optical configuration, and illuminating the measurement surface according to a second illumination pattern while in the second optical configuration.

[0047] Thus, measuring the position of the laser light may be part of measuring the position of the illumination pattern and / or measuring the positions of one or more predetermined points in the illumination pattern. For example, if the illumination pattern includes two intersecting lines (e.g., a cross), the position of the intersection point may be determined. Generally, measuring the first and / or second positions may include measuring the intersection point of two lines of the corresponding illumination pattern.

[0048] The first illumination pattern and the second illumination pattern may be projected onto at least one sensor positioned on the measurement surface.

[0049] The sensor may provide an electrical signal indicative of the illumination position on the sensor, which may be further evaluated by a control unit of the device. For example, the sensor may provide two-dimensional image data that may be further evaluated by the control unit to determine the respective positions. The sensor may be a two-dimensional sensor, such as a CCD sensor or a CMOS sensor.

[0050] The first and second radiation patterns may be projected or printed onto a foil or plate or powder layer positioned on the measurement surface.

[0051] Instead of positioning a sensor on the measurement surface, a powder, foil or plate may be positioned on the measurement surface. An illumination pattern is projected onto the powder, foil or plate, so that the illumination pattern can be observed, for example, by a camera during projection or by the human eye. It should be noted that irradiation with laser radiation does not permanently change the powder, foil or plate.

[0052] Instead of positioning a sensor on the measurement surface, a powder, foil, or plate may be positioned on the measurement surface. The irradiation pattern is burned into the powder, foil, or plate, so that the irradiation pattern becomes visible after irradiation has stopped. In other words, the laser light leaves a visually detectable pattern on the powder, foil, or plate, for example, via a color-changing structure, a visible structure, or a tactile structure. For example, the foil may be photosensitive, meaning that the laser reacts with the foil to cause a color change. Alternatively, the laser may burn holes into the foil, so that the irradiation pattern includes one or more holes. The plate may be coated or plated, and the laser light leaves marks on the coating or plating, for example, by burning the coating or plating. Such foils or plates used to calibrate laser light are well known in the art. The structure may be generated by irradiating one or more layers of powder material.

[0053] The method may further include, before the step of measuring the first position and before the step of measuring the second position, observing the first irradiation pattern and the second irradiation pattern with a human eye, and based on the observation, determining that the step of measuring the first position and the step of measuring the second position should be performed.

[0054] For example, the foil or plate may be visually observed. Only if the observer detects an anomaly in the illumination pattern, the first and second positions are measured. If the observer does not detect an anomaly, the optical unit is considered to be calibrated and no further investigation (i.e., by measuring the exact position of the laser light in each optical configuration) is necessary. For example, if the observer identifies an asymmetry in the illumination pattern, the observer may decide to perform the step of measuring the first and second positions. Alternatively, an illumination pattern that is quickly illuminated and easily observable by the human eye is first illuminated. If the observer determines that additional measurements are necessary, an illumination pattern optimized for detailed computer-assisted evaluation is illuminated.

[0055] The first illumination pattern may include a first circle and the second illumination pattern may include a second circle concentric with the first circle.

[0056] This illumination pattern may be space-saving. Furthermore, the deviation between different optical configurations (the deviation or deformation of one circle relative to another) may be easily determined by the human eye. The illumination pattern may include one circle per optical configuration, where the circles are concentric and at least three optical configurations (e.g., nine optical configurations) are considered.

[0057] The apparatus may include multiple optical units, each of which may illuminate a set of concentric circles.

[0058] Thus, for each optical unit, a set of concentric circles may be projected or printed onto a powder, foil or plate for example for visual and / or camera inspection.

[0059] The second focal spot size may be at least 1.5 times, at least 2 times, at least 3 times, at least 4 times, at least 5 times, at least 6 times, at least 7 times, or at least 8 times larger than the first focal spot size.

[0060] The focal spot size may be determined by the diameter of the focal spot at the focal point. In other words, the focal spot size may correspond to the beam waist. The beam spot size projected onto the measurement plane may depend on the focal spot size and defocus of the laser light.

[0061] The apparatus may be an apparatus for producing three-dimensional workpieces by selective laser sintering and / or selective laser melting.

[0062] In this case, the apparatus may comprise typical elements of such a machine, such as a processing chamber, a powder application device, a carrier movable along the z-axis within a build cylinder, etc.

[0063] The method may further include irradiating a contour of the layer of the three-dimensional workpiece with a first focal spot size and irradiating a core portion of the three-dimensional workpiece within the contour with a second focal spot size larger than the first focal spot size. A position of the laser light irradiating at least one of the contour and the core portion may be corrected by at least one correction value.

[0064] In this case, it can be ensured that the area within the contour (shell) is completely solidified by the laser light with the second focal spot size.

[0065] The method steps of the method according to the first aspect may be performed in the order shown. However, the order of the steps is not limited to the order shown. For example, the method may be performed in the following order: setting a first optical configuration, setting a second optical configuration, measuring a first position, measuring a second position, and determining at least one correction value. The above order of steps may be particularly useful when printing an irradiation pattern onto a foil or plate. In this case, the irradiation step may be performed first, followed by the measurement step. In another example, the corresponding position measurement may be performed during or immediately after setting the optical configuration.

[0066] If measurements are performed at multiple locations (within the scanning range of the optical unit), the method according to the first aspect may be performed at one location in the order shown before performing the method at the next location. Alternatively, the method may be performed in the order in which the first optical configuration is set up and the first position is measured at a series of locations or at all locations before the second optical configuration is set up and the second position is measured at a series of locations or at every location.

[0067] According to a second aspect, there is provided a computer program product which, when executed by a processor of an apparatus comprising at least one optical unit for generating laser light, the at least one optical unit comprising a plurality of optical elements, instructs the apparatus to perform the method according to the first aspect.

[0068] The computer program product may be stored on a computer readable carrier. [Brief explanation of the drawings]

[0069] Preferred embodiments of the present invention will now be described in more detail with reference to the accompanying schematic drawings. [Figure 1] FIG. 1 shows a schematic diagram of an apparatus for manufacturing three-dimensional workpieces that can be used to perform methods according to embodiments described herein. [Figure 2] FIG. 2 shows two exemplary optical configurations of an optical unit that may be used in accordance with embodiments of the present disclosure. [Figure 3] FIG. 3 shows a diagram illustrating the lens positions of the two lenses of the optical unit for different optical configurations. [Figure 4] Figure 4 shows a diagram for one optical unit, showing the lateral offset of the position of each laser beam for different optical configurations. [Figure 5] FIG. 5 shows a flowchart of a method according to one embodiment of the present disclosure. [Figure 6] Figure 6 shows a diagram for one optical unit, showing the lateral offset of the position of each laser beam for different optical configurations after at least one correction value has been applied. [Figure 7] 7 shows a diagram of an exemplary illumination pattern with concentric circles, where each circle represents one optical configuration. [Figure 8] 8(a) and (b) show the illumination patterns for calibrating the 12 optical units before calibration. DETAILED DESCRIPTION OF THE INVENTION

[0070] FIG. 1 shows a schematic diagram of an apparatus 10 for manufacturing a three-dimensional workpiece 8 suitable for carrying out a method according to the present disclosure. However, the present disclosure is not limited to a method carried out by the exact apparatus of FIG. 1 . The method may be carried out by any suitable apparatus. For example, such an apparatus may be an apparatus for selective laser melting or selective laser sintering, where one or more laser beams may be used to selectively irradiate and solidify subsequent layers of raw material powder. Furthermore, the calibration techniques of the present disclosure are not limited to apparatus for powder bed fusion bonding. Those skilled in the art will understand that the calibration techniques discussed in this disclosure can be applied to various laser processing techniques involving apparatuses with optical units that generate laser beams with different focal spot sizes.

[0071] In this disclosure, it is assumed that the technology of powder bed fusion is well known to those skilled in the art, and therefore the details of such technology will not be discussed in detail. The apparatus 10 for carrying out the process of selective laser melting described below may serve as an example. A typical feature of powder bed fusion is that raw material powder is applied in layers, and each layer is selectively irradiated and solidified to produce one layer of the workpiece to be manufactured. After removing excess powder and optional post-processing steps (e.g., removing one or more support structures), the final workpiece is obtained.

[0072] 1 shows an apparatus 10 for manufacturing three-dimensional workpieces 8 by selective laser melting. The apparatus 10 comprises a processing chamber 12. The processing chamber 12 is sealable from the ambient atmosphere, i.e., the environment surrounding the processing chamber 12. A powder application device 14 disposed within the processing chamber 12 functions to apply raw material powder onto a carrier 16. A vertical movement unit 32 is provided such that the carrier 16 can be vertically displaced and moved vertically downward as the workpieces 8 are built up layer by layer from the raw material powder on the carrier 16 as the build height of the workpieces 8 increases.

[0073] The movability of the carrier 16 by the vertical movement unit 32 is well known in the field of selective laser melting and will not be described in detail here. As an alternative to a movable carrier 16, the carrier 16 may be provided as a stationary (or fixed) carrier (particularly with respect to the vertical z-direction), where the irradiation unit 20 (see below) and the processing chamber 12 are configured to move upwards during the build process (i.e., as the build height of the workpiece 8 increases). Furthermore, both the carrier 16 and the irradiation unit 20 may be independently movable along the z-direction.

[0074] The carrier surface of the carrier 16 forms a horizontal plane (xy plane). The direction perpendicular to this plane is defined herein as the vertical or build direction (z direction). Thus, each top layer 28 of raw material powder extends in a plane parallel to the horizontal plane (xy plane) defined above.

[0075] The apparatus further includes a gas inlet 6 for supplying an inert gas (e.g., argon) into the processing chamber 12. A gas outlet (not shown) may be provided to implement a gas circuit to generate a continuous flow of gas through the processing chamber 12.

[0076] Furthermore, a camera 4 is disposed in the processing chamber 12 to observe the laser beam 2 irradiated by the optical unit 20 toward the powder bed during operation and / or to observe the irradiated area after irradiation by the laser beam 2. Furthermore, it is possible to observe only the thermal radiation of the generated molten pool by blocking the wavelength of the laser beam 2 with a respective optical filter. The camera 4 may be part of a molten pool observation device. In the embodiment shown in FIG. 1 , the camera 4 has its own optical system configured to generate an image of the top layer 28 of raw material powder. To this end, the camera 4 may include focusing and / or zoom optics. In particular, in the embodiment shown in FIG. 1 , the camera 4 is used to determine the position of the laser beam 2 on a measurement surface 42. The measurement surface 42 corresponds to the surface of the top layer 28 of raw material powder disposed on the carrier 16.

[0077] The apparatus 10 further comprises an optical unit 20 (also called an irradiation unit) for selectively irradiating the laser light 2 onto a top layer 28 of raw material powder applied on the carrier 16. By means of the optical unit 20, the raw material powder applied on the carrier 16 may be subjected to laser radiation in a site-selective manner depending on the desired shape of the workpiece 8 to be produced.

[0078] The optical unit 20 includes a scanning unit 22 configured to selectively irradiate the laser beam 2 onto the raw material powder coated on the carrier 16. The scanning unit 22 is controlled by a control unit (not shown) of the apparatus 10. The scanning unit 22 may include a single mirror that is tiltable about two orthogonal axes. Alternatively, the scanning unit 22 may include two tiltable mirrors, each configured to tilt about a corresponding axis. The tiltable mirrors may be, for example, galvanometer mirrors.

[0079] Laser light is supplied to the optical unit 20 from a laser light source 18. The laser light source 18 may be provided inside the optical unit 20, as shown in Figure 1, or may be provided external to the optical unit 20. If provided external to the optical unit 20, the laser light is generated by the laser light source 18 and directed to the optical unit 20 via an optical fiber 24. Alternatively, the laser light may be directed into the optical unit 20 through air or vacuum, for example, by using one or more mirrors.

[0080] Laser light from laser source 18 is directed towards scanning unit 22. Laser source 18 may comprise, for example, a diode-pumped ytterbium fiber laser emitting laser light at a wavelength of approximately 1070-1080 nm.

[0081] The optical unit 20 further comprises two lenses 34 and 36 configured to focus the laser beam 2 to a desired focal position 38 along the z-axis. In the embodiment shown in Figure 1, both lenses 34 and 36 have positive optical power. The lens 34 further upstream in the optical path is configured to collimate the laser beam emitted by the fiber 24 so that a parallel or substantially parallel laser beam is produced. The lens 36 further downstream in the optical path is configured to focus the collimated (or substantially collimated) laser beam to a desired z-position.

[0082] In this regard, lens 36 may be considered a focus lens, since movement of lens 36 along the optical path results in a shift of focal position 38 relative to the z-axis. Lens 34 may be considered a zoom lens, since movement of lens 34 along the optical path changes the focal spot size at focal position 38. However, as will be discussed in more detail with respect to FIG. 2, movement of both lenses 34 and 36 is required to change the focal spot size while maintaining the position of focal position 38.

[0083] It should be further noted that the arrangement of lenses 34 and 36 shown in Figure 1 is only one of many possible examples of optical arrangements within the optical unit. For example, instead of the single telescope arrangement shown in Figure 1, a dual telescope arrangement with two lenses per telescope may be provided. To this end, it should be noted that optical unit 20 may include any number of optical components, such as lenses that may be fixed or movable, e.g., movable along the optical axis. However, as noted above, at least two lenses are provided that are movable to perform the focusing and zooming functions.

[0084] It should be further noted that changing the diameter of the laser spot generated within the top layer 28 can also be achieved by moving the focal point 38 outside of the layer 28 so that an unfocused beam is incident on the layer 28. In other words, changing the diameter of the laser spot generated within the layer 28 can also be achieved by "simply" defocusing, i.e., by simply moving the focusing lens 36 without moving the zoom lens 34. This situation is illustrated in FIG. 1, where a defocused laser beam 2 impinges on the layer 28 and is defocused by the defocus 40. In other words, the focal point 38 of the laser beam 2 is not within the layer 28. In this situation, the laser beam 2 is not "focused" on the layer 28. When the measurement surface 42 is positioned within the layer 28, the laser beam 2 is not focused on the measurement surface 42.

[0085] Rather, the present disclosure is directed to situations in which the focal spot size at the focal position 38 actually changes, i.e., the beam waist of the focused laser light 2 changes. In a preferred embodiment, the focal position remains in the measurement plane 42. To achieve this, at least two lenses are moved, i.e., the two lenses 34 and 36 are moved. Furthermore, the spot size may also be changed by a combination of changing the beam waist (focal spot size) and the focal position (defocus).

[0086] Further details of the optical configuration for various focal spot sizes are shown in FIG.

[0087] In FIG. 2 , the solid lines represent the first optical configuration, while the dotted lines represent the second optical configuration. The first optical configuration generates a smaller laser spot than the second optical configuration. More precisely, the focal spot size 44 in the first optical configuration is smaller than the focal spot size 46 in the second optical configuration. In both configurations, the laser light 2 is focused onto the measurement surface 42 (e.g., the top layer 28 in FIG. 1 ). To move the optical unit 20 from the first optical configuration to the second optical configuration, both lenses 34 and 36 must be moved along the optical axis. Considering the above, the positions of both lenses 34 and 36 to provide the desired focal spot size within the desired measurement surface 42 can be predefined and stored in a memory of a control unit configured to control the movement of both lenses 34 and 36 (see also FIG. 3 ).

[0088] FIG. 2 further illustrates the problems that can occur when changing the focal spot size while maintaining the focal position 38 in the same measurement plane 42.

[0089] The lenses 34 and 36 may not be perfectly aligned with respect to the optical axis. In other words, the following discussion may consider an optical axis along the x-axis in FIG. 2 , where this optical axis is defined by the orientation of the fiber 18. However, the lenses 34 and 36 may not be perfectly aligned with respect to this optical axis. As a result, movement of the lenses 34 and 36 “along the optical axis” will not be perfectly parallel to the optical axis. Therefore, by moving the lenses 34 and 36 “along the optical axis,” the focal position 38 will not only be changed in the z-direction (if necessary), but also in the xy-plane, i.e., within the measurement plane 42. In other words, a lateral shift of the focal position 38 will occur. This problem is illustrated in FIG. 2 by the error angle 48 relative to the optimum value. In an ideal case, the error angle 48 would be zero, and all optical elements would be perfectly aligned with their optical axes coinciding.

[0090] In addition to the above effects, thermal effects (eg, thermal lensing) can contribute to the effect of lateral shift of the focal position 38 .

[0091] The resulting lateral displacement is indicated by reference numeral 50 .

[0092] For example, if the optical arrangement is optimized for an optical configuration associated with a magnification value of 1 (smallest laser focus size), the lateral deviations caused by all other optical configurations (associated with higher magnification values) may be taken into account with respect to the position of the laser light 2 at magnification value 1.

[0093] Therefore, the more the lenses 34, 36 are moved from the optical configuration for a magnification value of 1, the greater the expected lateral offset 50 (with respect to the laser position in the first optical configuration) since the zero point for determining the lateral offset 50 is defined relative to the optical configuration for a magnification value of 1.

[0094] Figure 3 shows the movement of the zoom lens 34 and the focus lens 36, where the lens position is normalized to zero for a magnification of 1. In Figure 3, the upper curve shows the lens position of the zoom lens 34, and the lower curve shows the lens position of the focus lens 36, both in arbitrary units. As can be seen from Figure 3, the maximum movement of both lenses occurs near a magnification of 2.

[0095] 4 shows the corresponding measured lateral offset for one exemplary optical unit 20 of the apparatus 10. The lateral offset in the xy plane (measurement plane) is divided into a component along the x-axis and a component along the y-axis. Furthermore, the offset of the optical configuration corresponding to a magnification value of 1 is normalized to zero.

[0096] From Figure 4, it can be seen that when different magnification values ​​(corresponding to different optical configurations) are set for the optical units, each optical unit will exhibit the above problem of lateral shift in focus position. As can be expected from the above considerations regarding the lens positions shown in Figure 3, the lateral offset (in both x and y directions) is greatest at a magnification value of approximately 2, around which the movement of both lenses is greatest.

[0097] In the following, methods aimed at eliminating or at least reducing the lateral offset shown in FIG. 4 are considered.

[0098] 5 illustrates a method for calibrating the position of a laser beam 2 in an apparatus 10 comprising at least one optical unit 20 for directing the laser beam 2, according to one embodiment of the present disclosure. The at least one optical unit 20 comprises a plurality of optical elements (such as lenses). As an example, the apparatus 10 discussed with respect to FIG. 1 may be used to implement the method of FIG. 5.

[0099] According to a first step 52, the method comprises setting a first optical configuration for a plurality of optical elements of at least one optical unit 20, thereby directing (in particular, focusing) the laser light 2 onto the measurement surface 42 with a first focal spot size 44. According to a second step 54, the method comprises measuring a first position in the measurement surface 42 of the laser light 2 generated with the first optical configuration, and preferably saving a first position data set based on the measured first position. According to a third step 56, the method comprises setting a second optical configuration for a plurality of optical elements of the at least one optical unit 20, thereby directing (in particular, focusing) the laser light 2 onto the measurement surface 42 with a second focal spot size 46 different from the first focal spot size 44. According to a fourth step 58, the method comprises measuring a second position in the measurement surface 42 of the laser light 2 generated with the second optical configuration, and preferably saving a second position data set based on the measured second position. According to a fifth step 60, the method includes determining at least one correction value based on the measured first position and the measured second position.

[0100] Steps 52 to 60 of this method may be performed in the order shown above. Thus, the position of the laser beam may be measured each time after the corresponding optical configuration is set and the corresponding laser beam is irradiated. However, this method may also be performed such that the optical configuration is first set and the measurement surface 42 is irradiated, and then the position is measured. In this case, the order of steps may be 52, 56, 54, 58, and 60.

[0101] According to the present calibration technique, there are various ways in which the laser light 2 can be directed onto the measurement surface 42 and the position at which it can be determined.

[0102] According to a first example, laser beam 2 is irradiated onto top powder layer 28, and the positions of the laser beam (i.e., the first and second positions) are determined by a camera, such as camera 4 in Figure 1. In this case, the positions may be determined either as laser beam 2 irradiates powder layer 28 (at an energy density high enough to melt the powder, high enough to temporarily heat the powder, or low enough to have no effect on the powder), or after laser beam 2 has burned a predetermined irradiation pattern into powder layer 28.

[0103] According to a second example, the sensor is positioned in the measurement plane 42, e.g., in the plane in which the top powder layer 28 is irradiated during the build process of the device 10, or in a parallel plane with a certain offset above or below the plane of the powder layer. For this purpose, the sensor may be positioned on the carrier 16 and moved upwards or downwards until it is positioned in the plane of the top powder layer 28. The sensor may be a two-dimensional sensor, such as a CCD sensor or a CMOS sensor. Alternatively, if a sensor is used, the sensor may directly output position information of the current irradiation or may output two-dimensional image data. Based on the two-dimensional image data, the position after irradiation is determined.

[0104] According to a third example, the laser beam 2 is irradiated onto a foil or plate forming the measurement surface 42. The foil or plate may be positioned in a plane where the top powder layer 28 is irradiated during the build process of the device 10. For this purpose, the foil or plate may be positioned on the carrier 16 and moved upward or downward until it is positioned in the plane of the top powder layer 28. The foil may be a photosensitive foil. The plate may, for example, comprise anodized aluminum. In a preferred case, the laser beam 2 leaves a visible mark on the foil or plate according to the irradiation pattern projected onto the foil or plate. After irradiation, the irradiation pattern may be observed by the human eye and / or a camera. Alternatively, the laser may only be projected onto the foil or plate and, for example, observed by a camera during projection. The position of the laser beam 2 may be determined with the aid of a computer, ie with the aid of image analysis software.

[0105] According to a fourth example, the sensor or foil or plate may be positioned in a side region of the processing chamber 12, where the laser light 2 is deflected by a movable mirror. To perform the measurement and calibration, the movable mirror is moved into the laser light 2. Once the measurement and calibration are finished, the movable mirror may be moved away from the laser light 2 again. In this case, the measurement plane may not be parallel to the xy plane, but may for example be parallel to the z axis. For example, the measurement plane may be the xz or yz plane.

[0106] According to a fifth example, the sensor or foil or plate may be positioned next to the top layer 28 of raw material powder, i.e., on the ground in the bottom region of the processing chamber 12. The plane on which the sensor or foil or plate is located may correspond to the plane of the top layer 28.

[0107] The following describes how the steps of the method of Figure 5 can be performed by the embodiment of apparatus 10 shown in Figure 1. According to the embodiment described below, the method is performed in the order of steps 52, 56, 54, 58, and 60.

[0108] Step 52: It is preferable to set a first optical configuration of the lenses 34 and 36 in the optical unit 20 so that the laser light 2 is focused on the measurement surface 42. The optical configuration refers to a predetermined position of the lenses 34 and 36 along the optical axis. To set the optical configuration, the lenses may be moved by corresponding actuators controlled by a control unit of the apparatus 10. The positions of the lenses 34 and 36 may be stored in the control unit, for example, in the form of a look-up table. To set the positions of the lenses 34 and 36, a desired magnification value (e.g., magnification value 1) may be selected, and the corresponding lens positions may be read from the look-up table. For example, FIG. 3 shows the relationship between the desired magnification value and the corresponding lens positions of the lenses 34 and 36. The position information of the lenses 34 and 36 may be updated, for example, after a calibration of the focal spot position 38 and / or the magnification value is performed. However, since the details of this calibration are well known to those skilled in the art, they will not be described in further detail herein.

[0109] As indicated above, there are various possibilities for implementing this method, particularly with regard to how the position of the laser beam 2 is measured. In the following, we will focus on the example using a photosensitive foil, according to the third example above. However, as will be understood by those skilled in the art, other possibilities according to the first to fifth examples can also be realized using the device 10 of FIG. 1.

[0110] The optical configuration is preferably set so that the laser focus position 38 is within a measurement plane 42 that is formed where the top layer 28 of feedstock is provided during the build process of the apparatus 10. To this end, a photosensitive foil is placed on the carrier 16, and the carrier is moved (up or down) until the foil is positioned at the desired measurement plane 42.

[0111] The scanning unit 22 is controlled such that a predetermined first illumination pattern is illuminated onto the foil while the illumination unit 20 is in the first optical configuration. Examples of possible illumination patterns are described with reference to Figures 7 and 8. For example, the illumination pattern of the first optical configuration may be a first circle having a first radius.

[0112] Step 56: After the irradiation pattern of the first optical configuration is completed, a second optical configuration is set in the optical unit 20. For example, a second magnification value (such as a magnification value of 5) may be selected, and the lenses 34 and 36 are moved to predetermined positions at the desired magnification value (e.g., based on the data shown in FIG. 3). The laser focus position 38 may be maintained within the measurement plane 42. In other words, as shown in FIG. 2, only the "zoom" of the laser light 2 may be changed, without changing its focus position 38. The focus position 38, and therefore the beam waist, may remain at the measurement plane 42. Alternatively, the desired magnification may be achieved by combining changes in the beam waist and focus position.

[0113] In this second optical configuration, a second radiation pattern is projected onto the foil, which may be a second circle concentric with the first circle but having a different (e.g., larger) radius.

[0114] In the following optional steps, additional optical configurations are set up and corresponding illumination patterns are printed onto the foil (eg, as concentric circles).

[0115] Furthermore, if the apparatus 10 includes multiple optical units 20, the other optical units may perform the same steps as discussed above for the optical unit 20 simultaneously or one after the other.

[0116] Steps 52 and 56 discussed above may be followed by an optional step in which the foil is inspected by the human eye of a person (e.g., the operator of the apparatus 10). If the person detects any irregularities in the applied illumination pattern (e.g., asymmetry, unexpected line thickness, unexpected discoloration, etc.), the decision may be made to further inspect the foil by performing detailed measurements of the laser position as discussed above. However, if the person does not detect any visually perceptible irregularities, the optical unit 20 of the apparatus is deemed sufficiently calibrated, and no further measurements are performed. In this case, the process of building the three-dimensional workpiece can begin. The inventors have discovered that an illumination pattern including concentric circles is advantageous for inspection by the human eye, as irregularities can be detected much more easily than, for example, a pattern of parallel stripes.

[0117] Step 54: The foil is observed by a camera, e.g., camera 4 of device 10 or an external camera, and a two-dimensional image is generated. Based on the two-dimensional image, the position of the first irradiation pattern is determined, for example, relative to a reference point on the foil (e.g., relative to a corner of the foil or relative to one or more reference marks burned into the foil). In this way, the position of the laser beam 2 when it is irradiated on the foil in the first optical configuration is determined. This position measuring process can be performed fully automatically by image analysis software, or an operator can inspect the recorded images and set the respective markers in the two-dimensional image data. A first position data set indicating the first position is saved.

[0118] Step 58: Measure the second position of the second optical configuration by measuring the position of the second illumination pattern in the same way as measuring the first position of the first optical configuration. Note that it may be sufficient to measure the relative positions of the first and second illumination patterns (rather than with respect to a "global" reference point on the foil). In this case, a calibration may be performed, for example, to normalize to the first optical configuration. A second position data set indicating the second position is saved.

[0119] In addition, if more than the first and second optical configurations are to be considered, the positions of additional illumination patterns are determined.

[0120] Step 60: Once the positions of all optical configurations have been measured, at least one correction value is determined based on the first and second position data sets. For example, a lateral offset (in the x-y plane) is determined between the expected laser position in the first optical configuration and the expected laser position in the second optical configuration. For example, the expected laser position may be the center of a circle corresponding to the respective illumination pattern. The offset may be expressed as x and y values. The offset is stored in the memory of the control unit of the device 10. For example, the offset value may be stored with respect to the position of the first optical configuration. In this case, the offset is not applied when using the first optical configuration. However, when the laser beam of the second optical configuration is illuminated, the offset is applied to the position data provided to the scanner unit 22 so that the desired position of the laser beam 2 in the x-y plane (measurement surface 42) corresponds to the illumination position. Similarly, offset values ​​for other optical configurations (e.g., a third optical configuration, a fourth optical configuration, etc.) may be stored in the control unit.

[0121] Alternatively, a global reference point on the foil (and thus in the measurement plane 42) may be used, and an offset value relative to the reference point may be applied to the entire optical configuration (i.e., also to the first optical configuration).

[0122] Furthermore, if an optical configuration is set up for illumination but the corresponding offset (correction value) is not stored in memory for this optical configuration, the correction value may be determined by interpolation, for example, assuming a linear dependency between the magnification value and the offset.

[0123] Figure 6 shows a diagram for the optical unit 20 after the correction values ​​have been applied. This diagram is almost identical to Figure 4 (which shows the situation before calibration). As can be seen from Figure 6, for the optical unit 20, it is possible to almost completely eliminate the lateral offset for different magnification values ​​(apart from unavoidable noise).

[0124] FIG. 7 shows an example of an illumination pattern for one optical unit 20 of the apparatus 10. The illumination pattern is illuminated by nine different optical configurations of the optical unit 20. First, vertical and horizontal lines forming a cross are drawn with a magnification of 1. In addition, a circle having a first radius is drawn with a magnification value of 1, where the center of the circle corresponds to the intersection of the vertical and horizontal lines. Then, successive circles are illuminated for each optical configuration, where every circle is concentric with the first circle. In the example of FIG. 7, the magnification values ​​used for the individual circles correspond to 1, 1.3, 1.5, 1.8, 2, 2.5, 3, 6, and 8.

[0125] As can be seen, the presence of irregularities in the example of Figure 7 is easily observable by the human eye, indicating the need for further detailed measurements.

[0126] The correction value for each optical configuration can be easily determined by comparing the positions of the intersections of the circle with the horizontal and vertical lines shown below the optical configuration with magnification value 1 based on the irradiation pattern shown in Figure 7.

[0127] Figure 8(a) shows the illumination patterns of 12 optical units, where each optical unit is provided with a set of concentric circles, as discussed above with respect to Figure 7. As can be seen directly from Figure 8(a) without detailed measurements, some of the circles are not perfectly concentric and appear to be misaligned in the xy plane. Therefore, corresponding optical units have a lateral offset that must be calibrated.

[0128] Figure 8(b) shows the illumination pattern after calibration, where no irregularities in the set of concentric circles are visually discernible.

[0129] The patterns shown in Figures 8(a) and 8(b) further include boxes, which are filled with hatch patterns from the corresponding optical units 20 to observe differences in refractive power (or defocus). If one or more of these white boxes have a different degree of discoloration than the other boxes, this is an indication that the laser output (or focus position) of the corresponding optical unit 20 is misaligned.

[0130] The patterns shown in Figures 8(a) and 8(b) further include a set of straight and curved lines, where each line is written by one optical unit 20 of the apparatus 10. This allows mismatches between the optical units (i.e., lateral shifts in the xy plane) to be easily detected and corrected, even if the individual lines do not form one continuous line.

[0131] The above measurements can be performed in a measurement mode of the apparatus 10, during setup of the apparatus 10, or during maintenance of the apparatus 10. During measurements, the apparatus 10 can be set to predetermined process parameters such as atmospheric oxygen content, temperature, etc. In particular, measurements can be performed under build conditions (apparatus 10 is heated, optical elements are heated, and gases are flowing as during the build process).

[0132] The techniques discussed above may allow for easy calibration of lateral misalignment that may occur when the optical unit is set to different optical configurations, thus improving the quality of the workpieces produced. The above-described embodiment can also be described as follows, but is not limited to the following. [Configuration 1] A method for calibrating a position of a laser beam (2) in an apparatus (10), the apparatus (10) comprising at least one optical unit (20) for directing the laser beam (2), the at least one optical unit (20) comprising a plurality of optical elements (34, 36), the method comprising: setting (52) a first optical configuration for the plurality of optical elements (34, 36) of the at least one optical unit (20) thereby directing (52) the laser light (2) onto a measurement surface (42) with a first focal spot size (44); measuring (54) a first position within the measurement plane (42) of the laser light (2) generated by the first optical configuration; setting (56) a second optical configuration of the plurality of optical elements (34, 36) of the at least one optical unit (20) to thereby direct the laser light (2) onto the measurement surface (42) with a second focal spot size (46) different from the first focal spot size (44); measuring (58) a second position within the measurement plane (42) of the laser light (2) generated by the second optical configuration; and determining (60) at least one correction value based on the measured first position and the measured second position. [Configuration 2] 2. The method of claim 1, wherein the step (52) of setting the first optical configuration for the plurality of optical elements (34, 36) of the at least one optical unit (20) and / or the step (56) of setting the second optical configuration for the plurality of optical elements (34, 36) of the at least one optical unit (20) includes a step of focusing the laser light (2) onto the measurement surface (42). [Configuration 3] storing a first position data set based on the measured first position; 3. The method of claim 1, further comprising: storing a second position data set based on the measured second position; The method, wherein the determining step (60) includes determining the at least one correction value based on the first position data set and the second position data set. [Configuration 4] 4. The method of any one of configurations 1 to 3, further comprising applying the at least one correction value during use of the apparatus (10) such that a relationship between a position of the laser beam in the first optical configuration and a position of the laser beam in the second optical configuration is known. [Configuration 5] 5. The method of any one of configurations 1 to 4, further comprising determining at least one correction value for a third optical configuration by performing interpolation or extrapolation based on the at least one correction value. [Configuration 6] 6. The method according to any one of configurations 1 to 5, wherein the step of setting the second optical configuration for the plurality of optical elements (34, 36) of the at least one optical unit (20) comprises a step of changing the position of at least two optical elements (34, 36), in particular at least two lenses (34, 36). [Configuration 7] illuminating the measurement surface (42) according to a first illumination pattern while in the first optical configuration; 7. The method of any one of configurations 1 to 6, further comprising illuminating the measurement surface (42) according to a second illumination pattern while in the second optical configuration. [Configuration 8] 8. The method of claim 7, wherein the first illumination pattern and the second illumination pattern are illuminated onto at least one sensor positioned on the measurement surface (42). [Configuration 9] 8. The method of claim 7, wherein the first radiation pattern and the second radiation pattern are projected or printed onto a foil or plate or a powder layer positioned on the measurement surface (42). [Configuration 10] 10. The method of claim 9, further comprising the steps of observing the first irradiation pattern and the second irradiation pattern with a human eye before the step of measuring the first position and before the step of measuring the second position, and determining based on the observation that the step of measuring the first position (52) and the step of measuring the second position (58) should be performed. [Configuration 11] 11. The method of any one of configurations 7 to 10, wherein the first irradiation pattern includes a first circle and the second irradiation pattern includes a second circle concentric with the first circle. [Configuration 12] 12. The method of claim 11, wherein the apparatus (10) comprises a plurality of optical units, and a set of concentric circles is irradiated onto each of the optical units. [Configuration 13] 13. The method of any one of configurations 1 to 12, wherein the second focal spot size (46) is at least 1.5 times, at least 2 times, at least 3 times, at least 4 times, at least 5 times, at least 6 times, at least 7 times, or at least 8 times larger than the first focal spot size (44). [Configuration 14] The apparatus (10) is an apparatus for producing a three-dimensional workpiece (8) by selective laser sintering and / or selective laser melting, and the method comprises: illuminating a layer profile of a three-dimensional workpiece (8) with the first focal spot size (44); irradiating a core portion of the three-dimensional workpiece (8) within the contour with the second focal spot size (46) that is larger than the first focal spot size (44); 14. The method according to any one of configurations 1 to 13, wherein a position of the laser light (2) irradiating at least one of the contour and the core portion is corrected by the at least one correction value. [Configuration 15] 15. A computer program product that, when executed by a processor of an apparatus (10) comprising at least one optical unit (20) for generating laser light (2), the at least one optical unit (20) comprising a plurality of optical elements (34, 36), instructs the apparatus (10) to perform the method of any one of configurations 1 to 14.

Claims

1. A method for calibrating a position of a laser beam (2) in an apparatus (10), the apparatus (10) comprising at least one optical unit (20) for directing the laser beam (2), the at least one optical unit (20) comprising a plurality of optical elements (34, 36), the method comprising: setting a first optical configuration for the plurality of optical elements of the at least one optical unit, thereby directing the laser light onto a measurement surface with a first focal spot size; measuring (54) a first position within the measurement plane (42) of the laser light (2) generated by the first optical configuration; setting a second optical configuration of the plurality of optical elements of the at least one optical unit to direct the laser light onto the measurement surface with a second focal spot size that is different from the first focal spot size, the second focal spot size being at least 1.5 times larger than the first focal spot size; measuring (58) a second position within the measurement plane (42) of the laser light (2) generated by the second optical configuration; and determining (60) at least one correction value based on the measured first position and the measured second position.

2. 2. The method of claim 1, wherein the step of setting the first optical configuration for the plurality of optical elements of the at least one optical unit and / or the step of setting the second optical configuration for the plurality of optical elements of the at least one optical unit comprises focusing the laser light onto the measurement surface.

3. storing a first position data set based on the measured first position; 3. The method of claim 1, further comprising: storing a second position data set based on the measured second position; The method, wherein the determining step (60) includes determining the at least one correction value based on the first position data set and the second position data set.

4. 4. The method of claim 1, further comprising applying the at least one correction value during use of the apparatus (10) such that a relationship between a position of the laser light in the first optical configuration and a position of the laser light in the second optical configuration is known.

5. 5. The method of claim 1, further comprising determining at least one correction value for a third optical configuration by performing interpolation or extrapolation based on the at least one correction value.

6. 6. The method according to claim 1, wherein the step of setting the second optical configuration for the plurality of optical elements (34, 36) of the at least one optical unit (20) comprises changing the positions of at least two optical elements (34, 36), in particular at least two lenses (34, 36).

7. illuminating the measurement surface (42) according to a first illumination pattern while in the first optical configuration; The method of any one of claims 1 to 6, further comprising illuminating the measurement surface (42) according to a second illumination pattern while in the second optical configuration.

8. The method of claim 7, wherein the first and second illumination patterns are projected onto at least one sensor positioned on the measurement surface (42).

9. 8. The method of claim 7, wherein the first radiation pattern and the second radiation pattern are projected or printed onto a foil or plate or a powder layer positioned on the measurement surface (42).

10. 10. The method of claim 9, further comprising the steps of: observing the first irradiation pattern and the second irradiation pattern with a human eye before the step of measuring the first position and before the step of measuring the second position; and determining based on the observation that the steps of measuring the first position (52) and measuring the second position (58) should be performed.

11. 11. The method of claim 7, wherein the first illumination pattern comprises a first circle and the second illumination pattern comprises a second circle concentric with the first circle.

12. 12. The method of claim 11, wherein the device (10) comprises a plurality of optical units, and a set of concentric circles is irradiated onto each of the optical units.

13. 13. The method of any one of claims 1 to 12, wherein the second focal spot size (46) is at least 2 times, at least 3 times, at least 4 times, at least 5 times, at least 6 times, at least 7 times, or at least 8 times larger than the first focal spot size (44).

14. The apparatus (10) is an apparatus for producing a three-dimensional workpiece (8) by selective laser sintering and / or selective laser melting, and the method comprises: illuminating a layer profile of a three-dimensional workpiece (8) with said first focal spot size (44); irradiating a core portion of the three-dimensional workpiece (8) within the contour with the second focal spot size (46) that is larger than the first focal spot size (44); The method according to any one of claims 1 to 13, wherein a position of the laser light (2) irradiating at least one of the contour and the core portion is corrected by the at least one correction value.

15. 15. A computer program product that, when executed by a processor of an apparatus (10) comprising at least one optical unit (20) for generating laser light (2), said at least one optical unit (20) comprising a plurality of optical elements (34, 36), instructs the apparatus (10) to perform the method of any one of claims 1 to 14.

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