Method and device for calibrating an illumination system, computer program product and apparatus for producing a three-dimensional workpiece - Patents.com

Abstract

A method of calibrating an illumination system (10) for use in an apparatus (100) for producing three-dimensional workpieces is described. The method includes step (i) of setting a distance between a calibration plane (30) and an optical unit (16) of the illumination system (10) to a first distance (z1) in a z-direction perpendicular to the calibration plane (30). In step (ii), a first calibration pattern (p) is scanned using a scanner mirror (22) of the optical unit (16) that is positioned at a first base angular position while maintaining the distance between the calibration plane (30) and the optical unit (16) at the first distance (z1). 1,1 ) is projected onto a first xy region (a1) in the calibration plane (30). A second calibration pattern (p 2,1 ) is projected onto a second xy region (a2) in the calibration plane (30). Here, in the second basic angular position, the scanner mirror (22) is pivoted by at least ±1 degree with respect to the first basic angular position. In step (iii), the distance in the z direction between the calibration plane (30) and the optical unit (16) is set to a second distance (z2) different from the first distance (z1). In step (iv), a third calibration pattern (p) is projected onto a second xy region (a2) in the calibration plane (30). Here, in the second basic angular position, the scanner mirror (22) is pivoted by at least ±1 degree with respect to the first basic angular position. In step (iv), the distance between the calibration plane (30) and the optical unit (16) is set to a second distance (z2) different from the first distance (z1). 1,2 ) is irradiated onto the first xy region (a1), and a fourth calibration pattern (p 2,2 ) is irradiated onto the second xy region (a2). In step (v), the first calibration pattern (p 1,1 ), the second calibration pattern (p 2,1 ), the third calibration pattern (p 1,2 ), and the fourth calibration pattern (p 2,2 ) is evaluated to determine a focal position of the radiation beam (14) in the z direction as a function of the x,y coordinates in the calibration plane (30). In step (vi), the illumination system (10) is calibrated based on the determined focal position of the radiation beam (14).

Description

[Technical field]

[0001] The present invention relates to a method and a device for calibrating an illumination system for use in an apparatus for producing three-dimensional workpieces by irradiating a raw material powder layer with a radiation beam. Furthermore, the present invention is directed to a computer program product, which includes a program portion that performs the method for calibrating an illumination system when the computer program is executed on one or more computing devices. Finally, the present invention is directed to an apparatus for producing three-dimensional workpieces by irradiating a raw material powder layer with a radiation beam.

[0002] Powder bed fusion is an additive layering process by which powdered, in particular metallic or ceramic, raw materials can be processed into three-dimensional workpieces of complex shape. For this purpose, a raw material powder layer is applied onto a carrier and subjected to electromagnetic or particle radiation in a site-selective manner according to the desired geometry of the workpiece to be produced. The radiation penetrating the powder layer causes heating and, consequently, melting or sintering of the raw material powder particles. Further raw material powder layers are applied successively to the layer already subjected to laser treatment on the carrier until the workpiece has acquired the desired shape and size. Powder bed fusion can be used in particular for producing prototypes, tools, replacement parts or medical prostheses, such as dental or orthopedic prostheses, based on CAD data.

[0003] An illumination system which can be employed in an apparatus for producing three-dimensional workpieces by irradiating powdered raw material is described, for example, in EP 2 335 848. The illumination system comprises a radiation source, in particular a laser source, and an optical unit, which is supplied with the radiation beam emitted by the radiation source and which comprises a beam expander, a scanner unit and an objective lens designed in the form of an f-theta lens.

[0004] The scanner unit typically comprises a scanner mirror pivotable about a pivot axis for deflecting the radiation beam emitted by the radiation source of the irradiation system to a desired irradiation spot on the raw material powder layer. By using a pivotable scanner mirror, the focal plane of the deflected radiation beam is curved, while the irradiation plane, which may be defined, for example, by a plane or surface of the raw material powder layer to be irradiated, is typically flat. Thus, there is a defocus in the direction perpendicular to the irradiation plane (z direction) depending on the pivot angle of the scanner mirror and thus on the irradiation position in the irradiation plane, i.e. the xy coordinates of where the radiation beam impinges on the irradiation plane. The defocus can be compensated either with the aid of flat field optics or by focus tracking using suitable adjustable optical elements.

[0005] However, the defocus compensation achieved by known defocus compensation means may be insufficient, e.g. due to inaccuracy or insufficiency of the compensation or due to the presence of defocus-influencing optical elements that are not taken into account in the defocus compensation. Furthermore, if the defocus compensation is based on a focus position measurement that includes an adjustment of the focus position by scanner optics, this adjustment itself may deteriorate the accuracy of the defocus compensation. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] European Patent No. 2335848 Summary of the Invention

[0007] The present invention is directed to the object of providing a method, a device and a computer program product for accurately calibrating an illumination system used in an apparatus for producing a three-dimensional workpiece by irradiating a raw material powder layer with a radiation beam.Furthermore, the present invention is directed to the object of providing an apparatus for producing a three-dimensional workpiece by irradiating a raw material powder layer with a radiation beam emitted by an illumination system that can be accurately calibrated.

[0008] The invention is defined in the independent claims. Preferred embodiments of the invention are outlined in the dependent claims.

[0009] The present disclosure relates to a method for calibrating an illumination system used in an apparatus for producing a three-dimensional workpiece by irradiating a layer of raw material powder with a radiation beam emitted by the illumination system. The radiation beam emitted by the illumination system may be an electromagnetic radiation beam or a particle radiation beam. Furthermore, the illumination system calibrated using the methods described herein may be a multi-beam illumination system configured to emit multiple radiation beams.

[0010] The illumination system may comprise a radiation source, in particular a laser source, for example a diode-pumped ytterbium fiber laser. The illumination system may also comprise only a single radiation source. It is also conceivable for the illumination system to comprise a plurality of radiation sources, in particular if the illumination system is designed in the form of a multi-laser illumination system. Furthermore, the illumination system comprises an optical unit. This optical unit comprises a scanner with a pivotable scanner mirror adapted to scan the radiation beam across the raw material powder layer to be irradiated while varying and adapting the focal position of the radiation beam both in the direction of the beam path and in a plane perpendicular to the beam path. In addition to this scanner, the illumination system, in particular the optical unit, may comprise a beam expander for expanding the radiation beam emitted from the radiation source and an objective lens, in particular an f-theta objective lens. Alternatively, the illumination system may comprise a beam expander including focusing optics.

[0011] The method for calibrating an illumination system comprises the step (i) of setting a distance in a z-direction perpendicular to the calibration plane between a calibration plane and an optical unit of the illumination system to a first distance. The surface area and / or contour of the calibration plane may correspond to the surface area and / or contour of an illumination plane illuminated with a radiation beam during normal operation of the apparatus. However, the calibration plane may be offset in the z-direction relative to the illumination plane. The distance between the calibration plane and the optical unit may be measured between the calibration plane and a housing of the optical unit.

[0012] In step (ii), while maintaining a first distance between the calibration plane and the optical unit, a first calibration pattern is projected onto a first xy region in the calibration plane with a scanner mirror of the optical unit disposed at a first base angular position, and a second calibration pattern is projected onto a second xy region in the calibration plane with a scanner mirror of the optical unit disposed at a second base angular position, where the scanner mirror is pivoted by at least ±1 degree relative to the first base angular position.

[0013] Then, in step (iii), a distance between the calibration plane and the optical unit of the illumination system in the z-direction perpendicular to the calibration plane is set to a second distance different from the first distance. In step (iv), while maintaining the distance between the calibration plane and the optical unit at the second distance, a third calibration pattern is illuminated in a first xy region in the calibration plane using a scanner mirror of the optical unit arranged at a first basic angular position. Furthermore, a fourth calibration pattern is illuminated in a second xy region in the calibration plane using a scanner mirror of the optical unit arranged at a second basic angular position in which the scanner mirror is pivoted by at least ±1 degree relative to the first basic angular position.

[0014] To set the distance between the calibration plane and the optical unit of the illumination system in steps (i) and (iii), the optical unit can be moved relative to the calibration plane in a z-direction perpendicular to the calibration plane. This can be achieved by moving the entire illumination system or only the optical unit relative to the calibration plane, for example by displacing a carrier structure of the illumination system or the optical unit. However, preferably, the distance between the calibration plane and the optical unit of the illumination system is set by moving the calibration plane in a z-direction perpendicular to the optical unit of the illumination system.

[0015] In the context of the present application, the expression "xy-area" defines an area of ​​the calibration plane that is small relative to the entire calibration plane. In particular, each xy-area defined in the calibration plane has a surface area of ​​less than 5%, preferably less than 1%, more preferably less than 0.5%, in particular less than 0.2% of the surface area of ​​the entire calibration plane. Furthermore, the entire surface area of ​​each xy-area can be illuminated by pivoting the scanner mirror by less than ±0.75 degrees, preferably less than ±0.6 degrees, more preferably less than ±0.5 degrees, in particular less than ±0.3 degrees, about an initial angular position that defines the point of incidence of the radiation beam at the center of the xy-area.

[0016] In the context of the present application, the expression "basic angular position" defines a small range of angular positions that the scanner mirror takes to illuminate one of said xy regions. Each "basic angular position" may vary around an initial angular position that defines the point of incidence of the radiation beam at the center of the xy region within an angular range of less than ±0.75 degrees, preferably within an angular range of less than ±0.6 degrees, more preferably within an angular range of less than ±0.5 degrees, in particular within an angular range of less than ±0.3 degrees. Thus, typically, the change in the "basic angular position" of the scanner mirror when illuminating the xy region is smaller than the angular difference between the "first basic angular position" and the "second basic angular position". That is to say, the dimensions of the xy region in the x and y directions are typically smaller than the distance between the centers of the different xy regions. Furthermore, the xy regions do not overlap.

[0017] In step (v), the first, second, third and fourth calibration patterns are evaluated to determine a focal position of the radiation beam in a z-direction perpendicular to the calibration plane as a function of the x-y coordinates in the calibration plane. Finally, in step (vi), the irradiation system is calibrated based on the determined focal positions of the radiation beam. Preferably, this calibrating is performed to optimize the focusing of the radiation beam on an irradiation plane on which the radiation beam is incident during normal operation of the apparatus for producing three-dimensional workpieces. For example, the focal position of the radiation beam may be optimized in the calibrating step (vi) for an irradiation plane defined by a surface of a raw material powder layer that is selectively irradiated during normal operation of the apparatus for producing three-dimensional workpieces.

[0018] Thus, the method described herein not only allows to determine the focal position in the z direction perpendicular to the calibration plane, but also allows to associate different focal positions in the z direction with different xy coordinates in the calibration plane. For example, evaluation of the calibration pattern may show that in a first xy region the focal position is located closer to the scanner mirror than in a second region, or vice versa. Corresponding corrections can then be calculated and the illumination system calibrated, taking into account the changes in the focal position both in the z direction and in the xy plane.

[0019] Calibration of the illumination system according to the method described herein may be performed whilst implementing conventional defocus compensation, e.g. with the aid of flat-field optics or by focus tracking, in order to correct the defocus in the z-direction due to the curved focal plane of the radiation beam deflected by the pivotable scanner mirror, whereby the calibrating performed in step (vi) serves as a kind of fine adjustment in the z-direction of the focal position of the radiation beam as a function of the xy-coordinates of the point of incidence of the radiation beam on the illumination plane during normal operation of the apparatus for producing the three-dimensional workpiece.

[0020] However, it is also conceivable to carry out the method without any kind of defocus compensation being performed or in situ, in which case the calibrating performed in step (vi) comprises a correction of defocus in the z-direction caused by a curved focal plane of the radiation beam deflected by the pivotable scanner mirror.

[0021] Essentially, the calibration plane positioned in one of steps (i) or (iii) may coincide with an illumination plane in which the radiation beam impinges during normal operation of the apparatus for producing the three-dimensional workpiece, although preferably the calibrating in step (vi) is performed to optimize the focusing of the radiation beam at the illumination plane while taking into account a shift in the z-direction between the calibration plane and the illumination plane.

[0022] Steps (ii) and (iv) to (vi) may be performed for multiple radiation beams. Thus, if the illumination system is designed in the form of a multiple beam illumination system, a corresponding multiple calibration patterns may be illuminated in each xy region in the calibration plane by multiple radiation beams. For this purpose, each xy region may be subdivided into sub-regions, each of which is associated with one of the multiple radiation beams. The calibration patterns may be generated either by using multiple radiation beams simultaneously or one after the other.

[0023] The optical properties, such as the refractive index or the geometry of the optical fibers, lenses or other optical elements of the illumination unit, in particular the radius of curvature of the lenses forming the optical elements of the illumination unit, may change depending on the operating temperature of the illumination unit. Typically, these temperature-induced changes in the refractive index of the optical materials used to manufacture the optical elements of the illumination unit and the temperature-induced changes in the geometry of the optical elements of the illumination unit, such as the radius of curvature, lead to a shift in the focal position of the radiation beam emitted by the illumination unit. In particular, as the operating temperature of the illumination unit increases, the focal position of the radiation beam may be gradually shifted along the beam path of the radiation beam, and thus in the z-direction perpendicular to the calibration plane.

[0024] Thus, steps (ii) and (iv)-(vi) may be performed using irradiation systems with multiple different temperatures. Thus, the thermal focus shift may be determined and taken into account in steps (v) and (vi). To determine the temperature-dependent thermal focus shift, each xy-region may be subdivided into subregions, each of which is associated with one of multiple different temperatures. Basically, steps (ii) and (iv)-(vi) may be performed using irradiation systems with only two different temperatures. However, to determine the temperature-dependent thermal focus shift more accurately, it is preferred to perform steps (ii) and (iv)-(vi) using irradiation systems with more than two, for example six different temperatures.

[0025] To change the temperature of the irradiation system, the irradiation system may be heated by irradiating at least one heating area with a heating pattern prior to carrying out steps (ii) and (iv) to (vi). The heating pattern may include a 100 μm vector that may be irradiated during a cycle, for example, during 100 cycles. Furthermore, the heating pattern may be irradiated at an irradiation speed of about 1 mm / s. Irradiating a 100 μm vector during 100 cycles at an irradiation speed of about 1 mm / s may result in an irradiation time of about 10 seconds for one heating pattern. The heating pattern may also be point-shaped, whereby irradiation of the point-shaped heating pattern is performed with the radiation beam in a static deflection state.

[0026] Preferably, one heating area is associated with each calibration pattern, which is irradiated to heat the illumination system to the desired temperature. That is, the first heating area may be associated with the first calibration pattern, the second heating area with the second heating pattern, the third heating area with the third heating pattern, etc. On the one hand, each heating area should be located far enough away from its associated calibration pattern that the calibration pattern is not affected by this heating process. For example, a heating area should be located with respect to its associated calibration pattern such that the calibration pattern is not contaminated by smoke or other particulate impurities that arise when irradiating the heating pattern. However, on the other hand, each heating area should be located as close as possible to its associated calibration pattern in order to increase the accuracy of the determination of the thermal focus shift depending on the temperature and the xy coordinates, and consequently of the compensation thereof.

[0027] For example, a heating region may be positioned to have a distance of about 2 mm in one of the x- or y-directions relative to its associated calibration pattern. Alternatively or additionally, the region of the calibration plane extending from the heating region in the other of the x- or y-directions should not be covered by the associated calibration pattern, i.e., should be left unilluminated to create a kind of "trail" or "collider" for removing smoke or other particulate impurities that arise when the heating pattern is illuminated.

[0028] The irradiation system can be set to different temperatures by irradiating multiple heating patterns with different output powers of the radiation beam emitted by the irradiation system. The operating temperature of the irradiation unit strongly depends on the output power of the radiation beam emitted by the irradiation unit. Therefore, changing the output power of the radiation beam emitted by the irradiation system is a reliable means to change the temperature of the irradiation system. For example, multiple heating patterns can be irradiated with different output powers of the radiation beam of 100W, 200W, 300W, 400W, 500W, and 600W. When the heating patterns are irradiated with different irradiation powers, the irradiation time is preferably kept constant, for example set to 10 seconds.

[0029] The irradiation system may also be set to different temperatures by applying different heating patterns for different irradiation times. For example, the heating patterns may be applied for different irradiation times of 10 seconds, 12 seconds, 14 seconds, 16 seconds, 18 seconds, and 20 seconds. When the heating patterns are applied for different irradiation times, the output power of the radiation beam emitted by the irradiation unit is preferably kept constant, for example set to 500 W.

[0030] However, it is also conceivable to set the irradiance system to a number of different temperatures by irradiating a number of heating patterns with a number of different output powers of the radiation beam emitted by the radiation system and with different irradiation times. For example, in order to increase the temperature of the irradiance system, the output power of the radiation beam emitted by the irradiance system and simultaneously the irradiation time can be increased to promote heating of the irradiance system induced by the increase in the output power of the radiation beam emitted by the irradiance system.

[0031] Step (iii) may be repeatedly performed to position the calibration plane a plurality of further distances away from the optical unit of the illumination system, each of the plurality of further distances being different from the first and second distances. A plurality of further calibration patterns may be projected in the first and second xy-regions according to step (iv). The number of further calibration patterns projected in each of the first and second xy-regions preferably corresponds to the number of the plurality of further distances at which the calibration plane is positioned away from the optical unit of the illumination system.

[0032] While the distance between the calibration plane and the optical unit of the illumination system is maintained at at least one of the first distance, the second distance and a plurality of further distances, the at least one additional calibration pattern may be illuminated in an additional xy-region in the calibration plane, different from the first xy-region and the second xy-region. Preferably, a plurality of additional xy-regions may be defined in the calibration plane and illuminated to generate at least one additional calibration pattern in each of the additional xy-regions. The first xy-region, the second xy-region and the at least one additional xy-region are distributed across the calibration plane, allowing to determine focal positions of the radiation beam in differently spaced apart regions in the calibration plane.

[0033] Preferably, for each of the distances at which the calibration plane is disposed away from the optical unit of the illumination system, an additional calibration pattern is illuminated in at least one additional xy-region in the calibration plane. For example, the method may include arranging the calibration plane at 16 different distances from the optical unit of the illumination system. In addition to the first and third calibration patterns, 14 additional calibration patterns may then be illuminated in the first xy-region. Similarly, in addition to the second and fourth calibration patterns, 14 additional calibration patterns may then be illuminated in the second xy-region. The at least one additional xy-region is preferably illuminated to generate 16 additional calibration patterns, each of which is associated with one of the distances at which the calibration plane is disposed away from the optical unit of the illumination system.

[0034] A typical distance between the calibration plane and the optical unit is between 250 mm and 1000 mm, but in special cases it can be smaller or larger.

[0035] The scanner mirror in the second basic angular position can be pivoted relative to the first basic angular position by at least ±2 degrees, in particular by at least ±5 degrees, preferably by at least ±10 degrees, more preferably by at least ±15 degrees. By increasing the angular difference between the "first basic angular position" and the "second basic angular position", the distance between the centers of the different xy regions and, consequently, the surface area of ​​the calibration plane considered in the present calibration method can be increased.

[0036] In particular, the distance between the centre of the first xy region and the centre of the second xy region in the calibration plane may correspond to at least 15 times, in particular at least 100 times, preferably at least 500 times, and more preferably at least 1000 times, the diameter of the focused radiation beam.

[0037] An xy region may preferably have a size of at least 2 mm x 2 mm, in particular at least 4 mm x 4 mm, and less than 40 mm x 40 mm, in particular less than 25 mm x 25 mm, preferably less than 12 mm x 12 mm, which achieves high illumination speed and good readability using common optical evaluation methods.

[0038] The calibration plane may be defined by a surface or a plane of a raw material powder layer applied on a carrier. However, it is also conceivable that the calibration plane is defined by a surface of a burn-off film applied on a carrier. The carrier containing the raw material powder layer or the burn-off film may be a specific calibration plate that is placed in the beam path of the radiation beam during calibration of the irradiation system, but is removed after the calibration process is completed. The calibration plate containing the raw material powder layer or the burn-off film may be, for example, a glass plate or an aluminum rod. A metal frame may be placed on the glass plate. However, the raw material powder layer or the burn-off film may also be applied on a carrier, i.e. on a carrier that contains a raw material powder layer that is selectively irradiated by the radiation beam emitted by the irradiation system during normal operation of the device for producing three-dimensional workpieces. As a further alternative, the calibration plane may also be defined by a surface of a calibration plate that is placed in the beam path of the radiation beam during calibration of the irradiation system, but is removed after the calibration process is completed.

[0039] If the method for calibrating an illumination system includes heating the illumination system by irradiating a heating pattern in at least one heating area as described above, the burn-off film and / or the calibration plate should preferably be protected from undesired heating and especially heat-induced damage. The burn-off film and / or the calibration plate may therefore be provided with through holes in the area of ​​the at least one heating area, through which the radiation beam emitted by the illumination system can pass for heating the illumination system. The radiation beam may be captured by a radiation trap. For example, a metal frame arranged on the calibration plate may act as a radiation trap. This frame may be cooled, for example water-cooled, or may have a large thermal capacity to prevent overheating of the frame.

[0040] A calibration pattern generated in a calibration plane, for example by irradiating a raw material powder layer, a burn-off film or a calibration plate, can be detected by means of an optical detection device, in particular by means of a camera. The optical detection device may already record and thus detect the irradiation of the calibration pattern in situ while the calibration pattern is being irradiated onto the calibration plane. However, alternatively or additionally, it is also possible to use the optical detection device to record the generated calibration pattern after the irradiation of the calibration pattern onto the irradiation plane has been completed.

[0041] The optical detection device used for recording the calibration pattern may be an optical detection device already present in the apparatus producing the three-dimensional workpiece, e.g. for the purpose of monitoring the process, although, especially if an optical detection device is used for recording the completed calibration pattern, it may be a conventional camera, e.g. as employed in a handheld microscope or even as employed in a mobile phone camera.

[0042] Furthermore, a photosensitive sensor arrangement arranged above or below the calibration plane may be used for detection of the calibration pattern, again either already detecting the illumination of the pattern in situ and / or detecting the illumination of the pattern after illumination of the calibration pattern onto the calibration plane has been completed.

[0043] The calibration pattern may include at least one straight line. A line width of the line included in the calibration pattern may be evaluated and a focal position of the radiation beam in a z-direction perpendicular to the calibration plane may be determined based on the evaluation of the line width of the calibration pattern. For example, the line widths of lines of different calibration patterns generated in a selected xy region of the calibration plane may be compared and the calibration pattern having the thinnest straight line may be determined as the calibration pattern illuminated in the selected xy region with the radiation beam focused. However, the calibration pattern may also be evaluated using a greyscale analysis or other suitable image analysis to determine the focal position of the radiation beam in the z-direction.

[0044] In a preferred embodiment of the method for calibrating an illumination system, at least one of the calibration patterns comprises a plurality of preferably substantially parallel straight lines, whereby a line width analysis or other suitable image analysis is performed for each of the straight lines, and an average value is calculated from the individual analysis values ​​and can be used for further evaluation procedures. The straight lines may also extend radially relative to the optical center of the illumination system.

[0045] At least one of the calibration patterns may include a first block defined by a first plurality of substantially parallel straight lines and a second block defined by a second plurality of substantially parallel straight lines. The second plurality of substantially parallel straight lines of the second block may be arranged substantially perpendicular to the first plurality of substantially parallel straight lines of the first block. By evaluating the blocks of straight lines extending substantially perpendicular to each other, optical astigmatism in the xy plane, i.e. in the x or y direction, may be detectable and taken into account in the calibration of the illumination system.

[0046] The first plurality of substantially parallel straight lines of the first block may extend radially relative to the optical center of the illumination system. The second plurality of substantially parallel straight lines of the second block may then extend perpendicularly to the radial direction relative to the optical center of the illumination system. The straight lines of the calibration pattern extending radially relative to the optical center of the illumination system and illuminated with a radiation beam whose focal position is optimized across the calibration plane have a constant thickness across the calibration plane, since elliptical distortion is avoided. Thus, the calibration pattern including straight lines extending radially relative to the optical center of the illumination system further allows the determination of a beam diameter in dependence on the xy coordinates in the calibration plane.

[0047] The individual calibration pattern or the plurality of calibration patterns may include an identification marker indicating the position of the individual calibration pattern or the plurality of calibration patterns in the calibration plane. For example, the plurality of calibration patterns illuminated in a common xy region in the calibration plane may be provided with a common identification marker. The identification marker may be, for example, a bar code or other suitable marker allowing to identify the coordinates of the calibration pattern in the calibration plane. The identification marker may include one or more symbols, special characters, or easily identifiable marks that can be easily recognized by an automatic image processing algorithm. The identification marker may also include a code containing information of the data cells, in particular binary data cells. The data cells may be formed as light and dark marks, as the absence and presence of marks, or as marks of different colors and / or shapes. In a preferred embodiment, the data cells are formed as straight line marks with different lengths, e.g. a short straight line indicates a value of "0" and a long straight line indicates a value of "1". The data cells, in particular the straight lines, may be marked at regular intervals. Codes of straight lines of different lengths and equal distances have proven to be robust in recognition as well as in production. The presence of the identification markers allows the sub-areas of the calibration plane to be recorded and analyzed with high accuracy, while allowing the identification of the sub-areas in the calibration plane with the aid of the identification markers.

[0048] Additionally, the individual calibration pattern or patterns may be surrounded by a line or other suitable geometric structure, such as a dashed or dotted line. The line may be straight or curved. The line may define a "box" that contains the individual calibration pattern or patterns. The line may be used in performing the evaluation algorithm, thus simplifying the automatic evaluation of the calibration patterns.

[0049] The projection positions of the calibration patterns projected onto a common xy-area in the xy-area can be determined based on an xy-shift of the incidence points of the radiation beam on the calibration plane caused by a change in the distance in the z-direction between the calibration plane and the optical unit of the projection system. For example, this xy-shift can be compensated for when calculating the projection positions of the calibration patterns, so that the calibration patterns aligned in the xy-plane can be obtained independently of the change in the distance in the z-direction between the calibration plane and the optical unit during the calibration process. Such an approach can be useful when it is envisaged to detect the calibration patterns in situ. However, it is also conceivable to refrain from compensating for the xy-shift and instead use the xy-shift to generate calibration patterns arranged next to each other at a desired distance apart. Such an approach can be advantageous in instances where the calibration patterns are recorded only after the projection of all the calibration patterns onto the projection plane is completed.

[0050] The present disclosure also relates to a device for calibrating an irradiation system for use in an apparatus for producing a three-dimensional workpiece by irradiating a layer of raw material powder with a radiation beam emitted by the irradiation system, said device comprising a control unit, said control unit comprising: (i) setting a distance in a z-direction perpendicular to the calibration plane between the calibration plane and an optical unit of the illumination system to a first distance; (ii) controlling the illumination system to illuminate a first calibration pattern onto a first xy region in the calibration plane using a scanner mirror of the optical unit disposed at a first base angular position while maintaining a first distance between the calibration plane and the optical unit, and to illuminate a second calibration pattern onto a second xy region in the calibration plane using a scanner mirror of the optical unit disposed at a second base angular position, wherein in the second base angular position the scanner mirror is pivoted by at least ±1 degree with respect to the first base angular position; (iii) setting a distance in a z-direction perpendicular to the calibration plane between the calibration plane and an optical unit of the illumination system to a second distance different from the first distance; (iv) controlling the illumination system to illuminate a third calibration pattern onto a first x-y region in the calibration plane using a scanner mirror of the optical unit disposed at a first base angular position while maintaining a second distance between the calibration plane and the optical unit, and to illuminate a fourth calibration pattern onto a second x-y region in the calibration plane using a scanner mirror of the optical unit disposed at a second base angular position in which the scanner mirror is pivoted by at least ±1 degree relative to the first base angular position; (v) evaluating the first calibration pattern, the second calibration pattern, the third calibration pattern, and the fourth calibration pattern to determine a focal position of the radiation beam in a z direction perpendicular to the calibration plane as a function of x, y coordinates in the calibration plane; and (vi) calibrating the illumination system based on the determined focal position of the radiation beam.

[0051] The control unit may be further configured to perform steps (ii) and (iv)-(vi) for a plurality of radiation beams. The control unit may be configured to perform steps (ii) and (iv)-(vi) using an irradiation system having a plurality of different temperatures.

[0052] In order to vary the temperature of the irradiation system, the control unit may be configured to heat the irradiation system by irradiating at least one heating region with a heating pattern prior to carrying out steps (ii) and (iv) to (vi).

[0053] The control unit may be configured to set the irradiance system to a plurality of different temperatures by irradiating a plurality of heating patterns at a plurality of different irradiance powers and / or for a plurality of different irradiance times.

[0054] The control unit is preferably configured to repeatedly perform step (iii) in order to position the calibration plane a number of further distances away from the optical unit of the illumination system, the further distances being different from the first and second distances, respectively. Furthermore, the control unit may be configured to control the illumination system to irradiate a number of further calibration patterns in the first xy-region and the second xy-region according to step (iv). The number of further calibration patterns irradiated in the first xy-region and the second xy-region, respectively, may in particular correspond to the number of further distances at which the calibration plane is located away from the optical unit of the illumination system.

[0055] The control unit may be configured to control the illumination system to illuminate at least one additional calibration pattern onto an additional xy region in the calibration plane, different from the first xy region and the second xy region, while a distance between the calibration plane and the optical unit is fixed and maintained at at least one of the first distance, the second distance, and a plurality of further distances.

[0056] The control unit may be configured to control the illumination system such that the scanner mirror in the second basic angular position is pivoted relative to the first basic angular position by at least ±2 degrees, in particular by at least ±5 degrees, preferably by at least ±10 degrees, more preferably by at least ±15 degrees. Alternatively or additionally, the control unit may be configured to control the illumination system such that a distance between a centre of the first xy-region and a centre of the second xy-region in the calibration plane corresponds to at least 15 times, in particular at least 100 times, preferably at least 500 times and more preferably at least 1000 times the diameter of the focused radiation beam.

[0057] The calibration plane may be defined by a surface or plane of a raw material powder layer applied on the carrier, by a surface of a burn-off film applied on the carrier and / or by a surface of a calibration plate. The burn-off film and / or the calibration plate may comprise a through-hole in the area of ​​the at least one heating zone, allowing the passage of a radiation beam emitted by the irradiation system for heating the irradiation system. The device may also comprise a radiation trap, positionable downstream of the through-hole in the beam path of the radiation beam.

[0058] The device may comprise one or more detection systems for detecting the calibration pattern. In particular, the device may comprise an optical detection device configured to record the illumination of the calibration pattern at its location while the calibration pattern is illuminated onto the calibration plane and / or after illumination of the calibration pattern onto the illumination plane is completed. Alternatively or additionally, the device may comprise a photosensitive sensor arrangement arranged above or below the calibration plane.

[0059] The calibration pattern may include at least one line, curve, point or circle. The control unit may be configured in step (v) to determine a focal position of the radiation beam in a z-direction perpendicular to the calibration plane based on an evaluation of a width of the line, point or circle of the calibration pattern.

[0060] At least one of the calibration patterns may include a first block defined by a first plurality of substantially parallel straight lines and a second block defined by a second plurality of substantially parallel straight lines. The second plurality of substantially parallel straight lines of the second block may be disposed substantially perpendicular to the first plurality of substantially parallel straight lines of the first block. The first plurality of substantially parallel straight lines of the first block may extend radially relative to an optical center of the illumination system.

[0061] The control unit may be configured to control the illumination system such that an individual calibration pattern or a plurality of calibration patterns, in particular a plurality of calibration patterns illuminated onto a common xy region within the calibration plane, are provided with an identification marker indicating the position of the individual calibration pattern or the plurality of calibration patterns within the calibration plane.

[0062] The control unit may be configured to determine the irradiation positions of multiple calibration patterns projected onto a common xy region within the xy region based on an xy shift of the incidence point of the radiation beam on the calibration plane caused by a change in the distance in the z direction between the calibration plane and an optical unit of the projection system.

[0063] The computer program product comprises program portions that, when the computer program product is executed on one or more computing devices, perform methods as outlined according to one or more of the example implementations described throughout this disclosure.

[0064] An apparatus for producing a three-dimensional workpiece by irradiating a layer of raw material powder with a radiation beam comprises an irradiation system and the above-mentioned device for calibrating the irradiation system and / or a computer-readable recording medium on which the above-mentioned computer program product is stored.

[0065] Preferred embodiments of the invention will now be described in more detail with reference to the accompanying schematic drawings. [Brief description of the drawings]

[0066] [Figure 1] FIG. 1 shows an apparatus for producing a three-dimensional workpiece by irradiating a bed of raw material powder with a radiation beam. [Diagram 2] FIG. 2 shows the curved focal plane of a radiation beam deflected by a scanner mirror of the optical unit employed in the apparatus depicted in FIG. [Diagram 3]FIG. 3 shows the xy displacement of the point of incidence of a radiation beam onto a plane that is located at different distances from the optical unit in the z direction perpendicular to the xy plane. [Figure 4] FIG. 4 shows an exemplary arrangement of a calibration pattern that is suitable to be evaluated to calibrate the illumination system of the apparatus depicted in FIG. [Diagram 5] FIG. 5 shows the calibration pattern of FIG. 4, but without the compensation for xy misalignment suggested in FIG. [Figure 6] FIG. 6 shows a diagram for estimating the line width of the calibration pattern depicted in FIG. [Figure 7] FIG. 7 shows a diagram for determining the focus position in the z-direction using a caustic fit. [Figure 8] FIG. 8 shows a sub-area on which an alternative calibration pattern is arranged that is preferably evaluated for calibrating an illumination system emitting two radiation beams. [Figure 9] FIG. 9 shows a diagram suggesting the focal position of two radiation beams in the z direction perpendicular to the calibration plane as a function of the x coordinate in the calibration plane. [Figure 10] FIG. 10 shows a further alternative calibration pattern disposed sub-area suitable for being evaluated to determine thermal focus shift. [Figure 11] FIG. 11 shows a smartphone image of a burn-off film that can be used to analyze the calibration pattern shown in FIG. [Figure 12] FIG. 12 shows a diagram suggesting the focal position of a radiation beam in the z-direction perpendicular to the calibration plane as a function of the output power of the radiation beam when irradiating a heating pattern. [Figure 13] FIG. 13 shows a further alternative calibration pattern disposed sub-area suitable for being evaluated to determine time-dependent thermal focus shift. [Figure 14]FIG. 14 shows a smartphone image of a burn-off film that can be used to analyze the calibration pattern shown in FIG. [Figure 15] FIG. 15 shows a diagram suggesting the focal position of the radiation beam in the z-direction perpendicular to the calibration plane as a function of the heating time. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0067] FIG. 1 shows an apparatus 100 for producing three-dimensional workpieces by an additive manufacturing process. The apparatus 100 comprises a carrier 102 and a powder application device 104 for applying raw material powder onto the carrier 102. The raw material powder may be a metal powder, but may also be a ceramic powder or a plastic material powder or a powder containing different materials. The powder may have any suitable particle size or particle size distribution. However, the powder is preferably processed to a particle size of less than 100 μm. The carrier 102 and the powder application device 104 are contained in a process chamber 106 that is sealable against the surrounding atmosphere. The carrier 102 is vertically displaceable in a build cylinder 108 so that the carrier 102 can be moved downwards with the increasing build height of the workpieces 110 as they are built up from the raw material powder in layers on the carrier 102. The carrier 102 may also be equipped with a heater or cooler.

[0068] The apparatus 100 further comprises an irradiation system 10 for selectively irradiating the raw material powder layer applied on the carrier 102 with laser radiation. In the embodiment of the apparatus 100 shown in Fig. 1, the irradiation system 10 comprises a radiation beam source 12 configured to emit a radiation beam 14. The radiation beam source 12 may be a laser beam source configured to emit a laser beam. Associated with the radiation beam source 12 is an optical unit 16 for guiding and processing the radiation beam 14 emitted by the radiation beam source 12. However, it is also conceivable that the irradiation system 10 is configured to emit two or more radiation beams.

[0069] The optical unit 16 comprises two lenses 18 and 20, which in the embodiment shown in Fig. 1 both have positive refractive power. The lens 18 is adapted to collimate the laser light emitted by the radiation beam source 12 to generate a collimated or substantially collimated radiation beam. The lens 20 is configured to focus the collimated (or substantially collimated) radiation beam 14 at a desired position in the z-direction. The optical unit 16 further comprises a pivotable scanner mirror 22, which serves to deflect the radiation beam 14 and scan it in the x- and y-directions across an illumination plane I, where the illumination plane I is typically defined by the surface of a raw material powder layer applied on a carrier 102 to be selectively illuminated during operation of the apparatus 100.

[0070] 1 is arranged in the process chamber 16 to observe the radiation beam 14 and / or to observe the irradiated area after irradiation with the radiation beam 14. The optical detection device 24 may be part of the melt pool observation device, but may also be a separate device.

[0071] The control unit 26 may be arranged to control only the operation of the irradiation system 10 or may also be arranged to control further components of the apparatus 100, such as the powder application device 104. The control unit 26 comprises a computer readable recording medium on which a computer program product including program code portions is stored.

[0072] A controlled gas atmosphere, preferably an inert gas atmosphere, is established in the process chamber 106 by supplying a protective gas into the process chamber 106 via the process gas inlet 112. After being directed through the process chamber 106 and across the layer of raw material powder applied on the carrier 102, the gas is exhausted from the process chamber 106 via the process gas outlet 114. The process gas may be recirculated from the process gas outlet 114 to the process gas inlet 112 and may be cooled or heated in doing so.

[0073] During operation of the apparatus 100 for producing a three-dimensional workpiece, a raw material powder layer is applied onto the carrier 102 by the powder application device 104. To apply the raw material powder layer, the powder application device 104 is moved across the carrier 102, for example under the control of the control unit 26. Then, again under the control of the control unit 26, the raw material powder layer is selectively irradiated by the irradiation device 10 according to the geometry of the corresponding layer of the workpiece 110 to be produced. This step of applying the raw material powder layer onto the carrier 102 and selectively irradiating the raw material powder layer according to the geometry of the corresponding layer of the workpiece 110 to be produced is repeated until the workpiece 110 reaches the desired shape and size.

[0074] The radiation beam 14 is scanned across the raw material powder layer according to a scan pattern defined by a control unit 26. As shown in Figure 2, the lens 20 can focus the radiation beam 14 onto the illumination plane I when the pivotable scanner mirror 22 is arranged at an angular position such that the radiation beam 14 is incident on the illumination plane I at an angle perpendicular to the illumination plane I, i.e. when the beam axis of the radiation beam 14 is parallel to the z direction.

[0075] However, when the scanner mirror 22 is pivoted to deflect the radiation beam 14 such that its beam axis no longer extends perpendicular to the illumination plane I, the length of the beam path is increased and therefore the focal position of the radiation beam 14, shown by the dashed line in Fig. 2, is no longer located in the illumination plane I but is shifted in the z-direction to a position above the illumination plane I. In particular, the illumination plane I is typically flat, while the focal position of the radiation beam 14 deflected by the pivotable scanner mirror 22 defines a curved focal plane F. Thus, the defocus in the z-direction depends on the pivot angle of the scanner mirror 22 and thus on the illumination position in the illumination plane I, i.e. on the xy coordinates at which the radiation beam 14 impinges on the illumination plane I.

[0076] Therefore, in the apparatus 100, the defocus in the z-direction of the radiation beam 14 is corrected, as indicated by the dotted lines in Figures 1 and 2, for example with the aid of flat-field optics or by focus tracking using suitable adjustable optical elements (not shown in the figures). However, the defocus compensation achieved by these conventional compensation means does not take into account any xy coordinate dependence of the focus position that is not related to the increase in the beam path length due to the deflection of the radiation beam 14 by the pivotable scanner mirror 22. Therefore, in order to enable a more accurate correction of the focus position of the radiation beam 14, in the apparatus 100 the illumination system 10 is calibrated as discussed below.

[0077] To calibrate the illumination system 10 of the apparatus 100, a calibration plate 28 is introduced into the beam path of the radiation beam 14 such that the surface of the calibration plate 28 extends parallel to an illumination plane I in which the radiation beam 14 impinges during normal operation of the apparatus 100, as shown diagrammatically in Figure 3. In particular, the calibration plate 28 is arranged on a surface of the carrier 102 which contains the raw material powder and the workpieces 110 during normal operation of the apparatus 100. The surface of the calibration plate 28 defines a calibration plane 30 onto which a calibration pattern is illuminated, as will be described further below.

[0078] As an alternative to the use of a calibration plate 28, a burn-off film may be applied to either the calibration plate or the carrier 102 to detect and record the calibration pattern projected onto the calibration plane, the calibration plane being defined by a surface of the burn-off film. As a further alternative, a layer of raw material powder may be applied onto the carrier 102, with a surface or plane of the raw material powder layer defining the calibration plane onto which the calibration pattern is projected.

[0079] In the apparatus 100, the optical detection device 24 is used in its place to record the projection of the calibration pattern while it is projected onto the calibration plane 30 defined by the surface of the calibration plate 28. However, it is also conceivable to use the optical detection device 24 to record and detect the calibration pattern only after the projection of the calibration pattern onto the calibration plane is completed. For example, the optical detection device 24 can take a photograph of the burn-off film after the projection in order to detect the calibration pattern projected onto its surface.

[0080] Additionally, calibration plate 28 may include a photosensitive sensor arrangement that may be embedded within calibration plate 28 and thus may be located below calibration plane 30 and / or above calibration plane 30. The photosensitive sensor arrangement may be capable of detecting and recording the calibration pattern illuminated onto calibration plane 30 in situ and / or after illumination of the calibration pattern is completed.

[0081] In step (i) of the method for calibrating the illumination system 10, the distance in the z-direction perpendicular to the calibration plane 30 between the calibration plane 30 and the optical unit 16 of the illumination system 10 is set to a first distance z1 as shown in Fig. 3. For example, the first distance z1 may be measured between the calibration plane 30 and the housing 17 of the optical unit 16. This is achieved by appropriately moving the carrier 102 together with the calibration plate 28 located thereon in the z-direction until the surface of the calibration plate 28 defining the calibration plane 30 is located the first distance z1 away from the optical unit 16.

[0082] Then, in step (ii), the distance between the calibration plane 30 and the optical unit 16 is maintained at a first distance z1, while the first calibration pattern p 1,1 is projected onto a first xy region in the calibration plane 30. In the following, the reference symbol “a i " is the calibration pattern "p i,j The first calibration pattern p is used to specify an xy region in the calibration plane 30 that is illuminated to generate a 1,1 When irradiating the first calibration pattern p, the scanner mirror 22 is disposed at the first basic angular position. 1,1 After the irradiation of the second calibration pattern p is completed, the scanner mirror 22 is pivoted by at least ±1 degree, in particular by at least ±2 degrees, in particular by at least ±5 degrees, preferably by at least ±10 degrees, and more preferably by at least ±15 degrees, relative to the first basic angular position to reach the second basic angular position. Then, the scanner mirror 22 is rotated to the second basic angular position while remaining at the second basic angular position. 2,1is irradiated onto a second xy region a2 in the calibration plane 30.

[0083] A pivoting movement of the scanner 22 of at least ±1 degree ensures that the first and second xy-regions a1, a2 are sufficiently separated from one another in the calibration plane 30. In particular, the distance between the centre of the first xy-region a1 and the centre of the second xy-region a2 in the calibration plane 30 corresponds to at least 15 times, in particular at least 100 times, preferably at least 500 times, more preferably at least 1000 times, the diameter of the focused radiation beam 14. Such a selection of the distance between the centre of the first xy-region a1 and the centre of the second xy-region a2 is suitable, for example, when the diameter of the focused radiation beam 14 is smaller than about 300 μm.

[0084] Then, in step (iii), the distance in the z-direction perpendicular to the calibration plane between the calibration plane 30 and the optical unit 16 of the illumination system 10 is set to a distance z2 different from the first distance z1. For example, as shown in Figure 3, the calibration plate 28 can be displaced in the z-direction by moving the carrier 102 vertically downwards, so as to increase the distance between the calibration plane 30 and the optical unit 16 from the first distance z1 to a second distance z2.

[0085] In step (iv), the distance between the calibration plane 30 and the optical unit 16 is maintained at a second distance z2 while the third calibration pattern p 1,2 is projected onto a first xy region a1 in the calibration plane 30 with the scanner mirror 22 again positioned at the first basic angular position. 1,2 After the irradiation of the fourth calibration pattern p is completed, the scanner mirror 22 is again pivoted by at least ±1 degree, in particular by at least ±2 degrees, in particular by at least ±5 degrees, preferably by at least ±10 degrees, and more preferably by at least ±15 degrees, relative to the first basic angular position, to reach the second basic angular position. 2,2 is illuminated onto a second xy region a2 in the calibration plane 30 with the scanner mirror 22 positioned at a second base angular position.

[0086] Step (iii) moves the calibration plane 30 at a number of further distances z from the optical unit 16 of the illumination system 10. f This is repeated to space the two objects apart by an additional distance z f are different from the first and second distances z1, z2, respectively. 1,j , p 2,j is irradiated in the first and second xy regions a1, a2 according to step (iv). Here, a further calibration pattern p 1,j , p 2,j is the additional distance z at which the calibration plane 30 is located away from the optical unit 16 of the illumination system 10. f For example, to generate the exemplary arrangement of calibration patterns shown in Figure 4, the calibration plane 30 is positioned 16 different distances away from the optical unit 16, and a total of 16 calibration patterns are illuminated in each of the first and second xy regions a1, a2.

[0087] Furthermore, the distance between the calibration plane 30 and the optical unit 16 is determined by a first and second distance as well as a number of additional distances z1, z2, z f While the calibration pattern p i,j However, an additional xy region a1, a2 in the calibration plane 30 is different from the first and second xy regions a1, a2. i In the exemplary arrangement of the calibration pattern shown in FIGS. i are defined in the calibration plane 30, and these additional xy regions a i , for each of the different distances at which the calibration plane 30 is positioned from the optical unit 16, an additional calibration pattern p i,j is irradiated.

[0088] All adjacent additional xy regions a iIt is not necessary to pivot the scanner mirror 22 by at least ±1 degree to illuminate the first and second xy regions a1, a2. Instead, the method can be performed in such a way that only the first and second xy regions a1, a2 are spaced apart from each other such that a pivoting movement of the scanner mirror 22 of at least ±1 degree is required to illuminate the first and second xy regions a1, a2, while the additional xy regions a1, a2 are spaced apart from each other such that a pivoting movement of the scanner mirror 22 of at least ±1 degree is required to illuminate the first and second xy regions a1, a2. i may be arranged closer to each other and / or to the first and second xy regions a1, a2. However, the xy regions a1, a2, a defined in the calibration plane 30 may be arranged closer to each other and / or to the first and second xy regions a1, a2. i should not overlap. Furthermore, the xy regions a1, a2, a i are preferably distributed across the calibration plane 30, for example as shown in FIG.

[0089] The scanner mirror 22 is positioned at the first, second, and any further base angular positions while scanning the desired calibration pattern p 1,1 , p 1,2 , p i,j However, the xy regions a1, a2, a i When irradiating the xy region a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12, a13, a14, a15, a16, a17, a18, a21, a22, a19, a23, a24, a25, a26, a27, a28, a29, a31, a32, a33, a34 i The pivotal movement of the scanner mirror 22 about the initial angular position defined at the center of the is limited to less than ±0.75 degrees, preferably less than ±0.6 degrees, more preferably less than ±0.5 degrees, and especially less than ±0.3 degrees. In this way, the xy regions a1, a2, a i are each small compared to the entire calibration plane 30, having a surface area of ​​less than 5%, preferably less than 1%, more preferably less than 0.5%, especially less than 0.2% of the total surface area of ​​the calibration plane 30.

[0090] In the exemplary calibration pattern arrangement according to FIG. 1,1 , p 2,1 , p 1,2 , p 2,2 , p 1,j , p 2,j , p i,jcontains three parallel straight lines extending radially with respect to the optical center of the illumination system 10. By evaluating multiple straight lines, the presence of errors or impurities occurring in the illumination can be compensated for, thus increasing the reliability of the calibration method and reducing the risk of measurement errors. Furthermore, the common xy regions a1, a2, a i Calibration pattern p is projected onto 1,1 , p 2,1 , p 1,2 , p 2,2 , p 1,j , p 2,j , p i,j The illumination arrangement of the calibration pattern p is determined based on the xy shift of the incidence point of the radiation beam 14 on the calibration plane 30 caused by the displacement of the calibration plane 30 in the z direction, as shown in FIG. 1,1 , p 2,1 , p 1,2 , p 2,2 , p 1,j , p 2,j , p i,j are arranged next to each other, while the calibration pattern p 1,1 , p 2,1 , p 1,2 , p 2,2 , p 1,j , p 2,j , p i,j appears distorted as shown in FIG. 5. However, the control unit 26 locates the calibration pattern p 1,1 , p 2,1 , p 1,2 , p 2,2 , p 1,j , p 2,j , p i,j The appearance of the calibration pattern p 1,1 , p 2,1 , p 1,2 , p 2,2 , p 1,j , p 2,j , p i,j A transformation calculation can be performed to correct for the xy shift so that the appearance of the image can be transformed into that of the original image, and vice versa.

[0091] In step (v), the first, second, third, fourth and further calibration patterns p 1,1 , p 2,1 , p 1,2 , p 2,2 , p 1,j , p 2,j , p i,j is evaluated by the control unit 26 and the focal position of the radiation beam 14 in the z direction perpendicular to the calibration plane 30 is determined as a function of the x and y coordinates in the calibration plane 30. In particular, the control unit 26 determines the focal position of the calibration pattern p 1,1 , p 2,1 , p 1,2 , p 2,2 , p 1,j , p 2,j , p i,j Based on the evaluation of the line width of the selected calibration pattern p, the focus position of the radiation beam 14 in the z direction is determined. As shown in FIG. 1,1 , p 2,1 , p 1,2 , p 2,2 , p 1,j , p 2,j , p i,j is carried out for each of the lines and an average value is calculated from the individual analyses and used for further evaluation procedures.

[0092] As shown in FIG. 7, the selected xy regions a1, a2, a i The calibration pattern p generated in 1,1 , p 2,1 , p 1,2 , p 2,2 , p 1,j , p 2,j , p i,j The line width values ​​of are plotted against the z coordinate, i.e., the position of the calibration plane 30 along the z direction. Then, the calibration pattern p 1,1 , p 2,1 , p 1,2 , p 2,2 , p 1,j , p 2,j , p i,j However, when the radiation beam 14 is focused, the selected xy regions a1, a2, a i Calibration pattern p illuminated on 1,1 , p2,1 , p 1,2 , p 2,2 , p 1,j , p 2,j , p i,j However, it is also conceivable to determine the focal position using the fitting of a function corresponding to a Gaussian beam profile. The analysis shown in FIG. 7 is carried out by fitting all xy regions a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12, a13, a14, a15, a16, a17, a18, a19, a20, a21, a22, a23, a24, a25, a26, a27, a28, a29, a30, a31, a32, a33, a34, a35, a36, a37, a38, a39, a40, a41, a42, a43, a44, a45, a46, a47, a48, a49, a50, a51, a52, a53, a54, a i The result is the position of the focus of the radiation beam 14 in the z direction, depending on the x,y coordinates in the calibration plane 30.

[0093] Finally, in step (vi), the illumination system 10 is calibrated based on the determined focal position of the radiation beam 14. In particular, this calibration is performed so as to optimize the focusing of the radiation beam 14 in an illumination plane I, at which the radiation beam 14 is incident during normal operation of the apparatus 100 for producing the three-dimensional workpiece 110.

[0094] The above steps (ii) and (iv)-(vi) may also be performed for multiple radiation beams 14. For example, the calibration pattern p i,j o,││ , p i,j o,⊥ However, each of the multiple radiation beams 14 is incident on a selected xy region a i where "o" designates an optical unit emitting a selected one of the radiation beams 14, and the symbols "||" and "⊥" designate a calibration pattern p i,j o,││ , p i,j o,⊥ FIG. 8 shows an example xy domain a i and the calibration pattern p i,j 1,││ , p i,j 1,⊥ , p i,j 2,││ , p i,j 2,⊥ is generated by two radiation beams 14. In particular, the xy region a shown in FIG. iis subdivided into two sub-areas, each of which is associated with one of the two radiation beams. i The calibration pattern p in the left part (the first sub-area enclosed by the left dotted rectangle) of i,j 1,││ , p i,j 1,⊥ is generated by a first radiation beam 14 with the aid of a first optical unit 16. On the other hand, in the xy region a i The calibration pattern p in the right part (the second sub-area enclosed by the right dotted rectangle) of i,j 2,││ , p i,j 2,⊥ is generated by a second radiation beam 14 with the aid of a second optical unit 16 .

[0095] Calibration pattern p i,j o,││ , p i,j o,⊥ Each of the at least one first block p is defined by, for example, three substantially parallel straight lines. i,j o,││ and at least one second block p, for example defined by three substantially parallel straight lines. i,j o,⊥ Here, the second block p i,j o,⊥ The line in the first block p i,j o,││ Specifically, the calibration pattern p i,j o,││ , p i,j o,⊥ Each of the first blocks p is defined by a plurality of, here 16, first blocks p each of which is defined by three substantially parallel straight lines. i,j o,││ and a plurality of, here 16, second blocks p each defined by three substantially parallel straight lines. i,j o,⊥ Here, the second block p i,j o,⊥ The line in the first block p i,jo,││ In FIG. 8, a plurality of first blocks p i,j o,││ is the xy domain a i , and is enclosed by the upper dashed rectangle. Meanwhile, a number of second blocks p i,j o,⊥ is the xy domain a i and is enclosed by the lower dashed rectangle.

[0096] Common xy area a i Calibration pattern p is projected onto i,j o,││ , p i,j o,⊥ is the calibration pattern p in the calibration plane 30 i,j o,││ , p i,j o,⊥ The identification marker 32 indicates the position of the calibration pattern p i,j o,││ , p i,j o,⊥ 1. It may be a bar code, which allows the determination of the coordinates of the calibration plane 30 and thus allows the recording and analysis of partial areas of the calibration plane 30 to be carried out with high precision.

[0097] Although the provision of the identification marker 32 has been described herein with reference to FIG. 8 showing a calibration pattern produced by multiple radiation beams 14, the identification marker 32 may be provided in a calibration pattern p produced by a single radiation beam 14. i,j For example, the calibration pattern p shown in FIG. i,j One or more of the may be marked with an identification marker.

[0098] FIG. 9 shows an example of an xy region a i The calibration pattern p shown in FIG. i,j o,││ , p i,j o,⊥1 shows a diagram in which the focal position in the z direction of the radiation beam 14 used to generate is plotted against the x direction. A corresponding diagram can also be plotted for the y direction. Moreover, it is of course also possible to plot the focal position against the xy plane.

[0099] The curves shown in the diagram of FIG. 9 are obtained by evaluating either the first or second straight line blocks extending perpendicularly to each other. Considering the position of the lenses in the optical unit 16 and the scanner mirror 22 during the movement, the astigmatism axis lies transverse to the xy plane. However, the pattern of straight lines rotates such that the radially extending straight lines are directed towards the illumination optical unit. As a result, the result of the measurement across the xy plane is an oscillatory wave pattern as a function of the xy coordinates of the astigmatized focal position. A large discrepancy between the focal positions determined for different straight line blocks suggests astigmatism of the optical elements used to generate the radiation beam 14. The actual astigmatism, i.e. the distance between the focal positions of both beam axes, can be measured at the maximum of the oscillation. The dots in the diagram indicate the values ​​of the average focal position determined by averaging the values ​​obtained for the first and second straight line blocks. The arrangement of points in the diagram shows that the focus positions at the edge of the calibration plane 30 are shifted a smaller distance away from the optical unit 16. This shift in focus positions can be corrected by the calibration performed in step (vi) in order to obtain an even distribution of focus positions across the illumination plane I.

[0100] The above steps (ii) and (iv)-(vi) may also be performed with the illumination system 10 having a number of different temperatures Tm, and thermally induced focus shifts may be determined and taken into account in steps (v) and (vi). FIG. 10 shows an example xy region with a calibration pattern p i,j Tm,││ , p i,j Tm,⊥is generated using an irradiation system set at six different temperatures Tm = T1, ..., T6. In particular, the xy region a in Fig. 10 i is subdivided into six sub-regions, each associated with a number (six) of different temperatures Tm. In Fig. 10, each sub-region is enclosed by a dashed rectangle.

[0101] Similar to the arrangement shown in FIG. 8, the xy domain a according to FIG. i Calibration pattern in p i,j Tm,││ , p i,j Tm,⊥ Each of the at least one first block p is defined by, for example, three substantially parallel straight lines. i,j Tm,││ and at least one second block p, for example defined by three substantially parallel straight lines. i,j Tm,⊥ Here, the second block p i,j Tm,⊥ The line in the first block p i,j Tm,││ Specifically, the calibration pattern p i,j Tm,││ , p i,j Tm,⊥ Each of the first blocks p is defined by a plurality of, here 16, first blocks p each of which is defined by three substantially parallel straight lines. i,j Tm,││ and a plurality of, here 16, second blocks p each defined by three substantially parallel straight lines. i,j Tm,⊥ Here, the second block p i,j Tm,⊥ The line in the first block p i,j Tm,││ 8, the calibration pattern p i,j Tm,││ , p i,j Tm,⊥ An identification marker 32 is provided to indicate the position of the

[0102] In order to change the temperature of the irradiation system 10, prior to carrying out steps (ii) and (iv) to (vi), the irradiation system 10 is subjected to a heating pattern p hm At least one heating area h m In the exemplary arrangement of FIG. i,j Tm,││ , p i,j Tm,⊥ One heating zone h for each m Each heating zone h m is its associated calibration pattern p i,j Tm,││ , p i,j Tm,⊥ From the calibration pattern p i,j Tm,││ , p i,j Tm,⊥ The first block p of each i,j Tm,││ and the second block p i,j Tm,⊥ , i.e., the calibration pattern p i,j Tm,││ , p i,j Tm,⊥ The heating zones h are spaced far enough apart that they are not affected by the heating process. m is the calibration pattern p i,j Tm,││ , p i,j Tm,⊥ The first block p of each i,j Tm,││ and the second block p i,j Tm,⊥ In addition, the heating area h m The area of ​​the calibration plane 30 extending in the y direction from i,j Tm,││ , p i,j Tm,⊥ This means that the heating pattern p hmThis is to create a kind of "trail" or "collider" to remove smoke or other particulate impurities that are generated when the heating area h m Each has its associated calibration pattern p i,j Tm,││ , p i,j Tm,⊥ , and are positioned sufficiently close that the temperature-dependent thermal focus shift can be determined with the desired accuracy.

[0103] Heating pattern p hm contains a 100 μm vector that is irradiated for 100 cycles with an irradiation speed of about 1 mm / s. In order to protect the burn-off film and / or the calibration plate for defining the calibration plane 30 from unwanted heating, in particular from heat-induced damage, the burn-off film and / or the calibration plate are provided with a heating area h m The irradiation system 10 is provided with a through hole therein, which allows the radiation beam 14 emitted by the irradiation system 10 to pass therethrough in order to heat the irradiation system 10. The radiation beam 14 can be captured by a radiation trap.

[0104] The operating temperature of the irradiation unit 10 strongly depends on the output power of the radiation beam 14 emitted by the irradiation unit 10. In the arrangement of FIG. 10, the irradiation system 10 includes a plurality of heating patterns p hm is set to a number of different temperatures Tm by irradiating it with a number of different output powers of the radiation beam 14 emitted by the irradiation system 10. In particular, a number of heating patterns p h1 ~p h6 are irradiated with different output powers of the radiation beam 14, which are 100 W, 200 W, 300 W, 400 W, 500 W, and 600 W, to heat the irradiation system 10 to six different temperatures T1 to T6. The irradiation time tm is kept constant and set to 10 seconds.

[0105] Calibration pattern p in Figure 10 i,j Tm,││ , p i,j Tm,⊥A smartphone image of a burn-off film that can be used to evaluate is shown in FIG. 11. In the image of FIG. 11, the calibration pattern p i,j Tm,││ , p i,j Tm,⊥ can be evaluated and the focal position of the radiation beam 14 in the z direction perpendicular to the calibration plane 30 can be calculated by multiplying the focal position of the calibration pattern p i,j Tm,││ , p i,j Tm,⊥ In particular, the line width of each heating region h m For the associated calibration pattern p i,j Tm,││ , p i,j Tm,⊥ The line widths of the straight lines in the calibration pattern p can be compared. i,j Tm,││ , p i,j Tm,⊥ is a calibration pattern p illuminated with the radiation beam 14 in a focused state. i,j Tm,││ , p i,j Tm,⊥ The image in FIG. 11 also shows that in the burn-off film, the heated region h m It is also shown that a through hole 34 provided in the burn-off film is enlarged during irradiation of the burn-off film, where the enlargement of the through hole 32 proceeds with an increase in the output power of the radiation beam 14.

[0106] The diagram of FIG. 12 results from the evaluation of the images shown in FIG. 11 and shows the heating pattern p hm 12 shows the focal position of the radiation beam 14 in the z-direction perpendicular to the calibration plane 30 as a function of the output power of the radiation beam 14 when irradiating the illumination system 10. In other words, the diagram in Fig. 12 shows the thermally induced focal shift as a function of the temperature Tm of the illumination system 10. This thermally induced shift in focal position can be corrected by the calibration performed in step (vi) in order to obtain an even focal position distribution over the illumination plane I.

[0107] The lower curve shown in the diagram of FIG. 12 is obtained by conventional caustic measurement, while the upper curve in the diagram of FIG. 12 is obtained by using the calibration pattern p i,j Tm,││ , p i,j Tm,⊥ The line width of the calibration pattern p is obtained by evaluating the line width of the calibration pattern p. i,j Tm,││ , p i,j Tm,⊥ This suggests that the thermal focus shift can be determined with reliable accuracy by evaluating the linewidth of the calibration pattern p i,j Tm,││ , p i,j Tm,⊥ However, line width assessment is cheaper, faster, and automated compared to caustic measurements, which require application-specific measurement devices, trained personnel, and machine downtime.

[0108] Furthermore, the first block p i,j Tm,││ and the focus position obtained by evaluating the second block p i,j Tm,⊥ The focus positions obtained by evaluating θ can be compared as described above with reference to Figure 9 to determine the astigmatism of the optical elements used to generate the radiation beam 14. Again, this focus position shift can be corrected by a calibration performed in step (vi) in order to obtain an even focus position distribution over the illumination plane I.

[0109] The arrangement of FIG. 13 shows that the irradiation system has multiple heating patterns p hm 10 in that the wafer is heated to six different temperatures Tm=T1, ..., T6 by irradiating the wafer with a plurality of different irradiation times, i.e., heating times, tm. In particular, the plurality of heating patterns p h1 ~p h6is irradiated with different irradiation times, namely heating times of 0, 0.5, 1, 3, 8 and 20 seconds, while the output power of the radiation beam 14 emitted by the irradiation unit 10 is kept constant and set to 500 W. hm again comprises a 100 μm vector, which is irradiated for 100 cycles with an irradiation speed of about 1 mm / sec. Alternatively, the arrangement of FIG. 13 corresponds to that of FIG.

[0110] FIG. 14 shows the calibration pattern p of FIG. i,j Tm,││ , p i,j Tm,⊥ 14 shows a smartphone image of a burn-off film that can be used to evaluate the calibration pattern p As described above with reference to FIGS. i,j Tm,││ , p i,j Tm,⊥ can be evaluated and the focal position of the radiation beam 14 in the z direction perpendicular to the calibration plane 30 can be calculated by multiplying the focal position of the calibration pattern p i,j Tm,││ , p i,j Tm,⊥ The image in FIG. 14 also shows that in the burn-off film, the heated region h m It shows that through-holes 34 in the burn-off film expand during irradiation. The expansion of through-holes 32 progresses with increasing heating time. No through-holes are provided in heating region h1 because the heating time in heating region h1 is 0 seconds.

[0111] Figure 15 shows a diagram obtained from evaluation of the images shown in Figure 14 and showing the focal position of the radiation beam 14 in the z direction perpendicular to the calibration plane 30 as a function of the irradiation / heating time tm. Again, this thermally induced shift in the focal position can be corrected by the calibration performed in step (iv) in order to obtain an even focal position distribution over the irradiation plane I.

Claims

1. 1. A method of calibrating an illumination system (10) for use in an apparatus (100) for producing a three-dimensional workpiece by irradiating a layer of raw material powder with a radiation beam (14) emitted by the illumination system (10), the method comprising: (i) determining a distance between a calibration plane (30) and an optical unit (16) of the illumination system (10) in a z-direction perpendicular to the calibration plane (30) as a first distance (z １ ) and (ii) changing the distance between the calibration plane (30) and the optical unit (16) to the first distance (z １ ), a first calibration pattern (p) is scanned using a scanner mirror (22) of the optical unit (16) disposed at a first basic angular position. １，１ ) in a first xy region (a １ ) and a second calibration pattern (p ２，１ ) in a second xy region (a ２ ), wherein in the second base angular position, the scanner mirror (22) is pivoted by at least ±1 degree relative to the first base angular position; (iii) determining the distance between the calibration plane (30) and the optical unit (16) of the illumination system (10) in the z direction perpendicular to the calibration plane (30) as the first distance (z １ ) different from the second distance (z ２ ) and (iv) changing the distance between the calibration plane (30) and the optical unit (16) to the second distance (z ２ ), a third calibration pattern (p １，２ ) in the first xy region (a １ ), and a fourth calibration pattern (p) is scanned using the scanner mirror (22) of the optical unit (16) that is disposed at the second basic angular position in which the scanner mirror (22) is pivoted by at least ±1 degree with respect to the first basic angular position. ２，２ ) in the second xy region (a ２ ) and (v) the first calibration pattern (p １，１ ), the second calibration pattern (p ２，１ ), the third calibration pattern (p １，２ ), and the fourth calibration pattern (p ２，２ ) to determine a focal position of the radiation beam (14) in the z direction perpendicular to the calibration plane (30) as a function of x, y coordinates in the calibration plane (30); (vi) calibrating the illumination system (10) based on the determined focal position of the radiation beam (14).

2. - steps (ii) and (iv) to (vi) are performed for a plurality of radiation beams (14); and / or The method of claim 1, wherein steps (ii) and (iv) to (vi) are carried out using the irradiation system (10) having a plurality of different temperatures (Tm).

3. In order to change the temperature of the irradiation system (10), the irradiation system (10) comprises, prior to carrying out steps (ii) and (iv) to (vi), at least one heating zone (h ｍ ) with heating pattern (p ｈｍ 3. The method of claim 2, wherein the heating is performed by irradiating the substrate with light.

4. The irradiation system (10) has a plurality of heating patterns (p ｈｍ 4. The method according to claim 2 or 3, wherein the substrate (10) is set to a plurality of different temperatures (Tm) by irradiating the substrate (10) with a plurality of different output powers of the radiation beam (14) emitted by the irradiation system (10) and / or for a plurality of different irradiation times.

5. - step (iii) comprises measuring the calibration plane (30) at a number of further distances (z ｆ ) apart, ｆ ) are each the first distance (z １ ) and the second distance (z ２ ) and - a number of further calibration patterns (p １，ｊ , p ２，ｊ ) according to step (iv), the first xy region (a １ ) and the second xy region (a ２ ) is irradiated to the first xy region (a １ ) and the second xy region (a ２ ) are respectively irradiated with the further calibration patterns (p １，ｊ , p ２，ｊ ) is determined in particular by the further distance (z ｆ 5. The method according to claim 1 , wherein the number of

6. The distance between the calibration plane (30) and the optical unit (16) of the illumination system (10) is the first distance (z １ ), the second distance (z ２ ), and a number of said further distances (z ｆ ) while being maintained at at least one of the calibration patterns (p ｉ，ｊ ) is the first xy region (a １ ) and the second xy region (a ２ ) in the calibration plane (30) different from the additional xy region (a ｉ 6. The method according to claim 1 , wherein the light is irradiated with light having a wavelength of 100 nm or less.

7. the scanner mirror (22) in the second basic angular position is pivoted relative to the first basic angular position by at least ±2 degrees, in particular by at least ±5 degrees, preferably by at least ±10 degrees and further preferably by at least ±15 degrees; and / or - the first xy region (a １ ) and the center of the second xy region (a ２ 7. The method according to claim 1 , wherein the distance between the centers of the first and second electrodes corresponds to at least 15 times, in particular at least 100 times, preferably at least 500 times, and more preferably at least 1000 times, a diameter of the focused radiation beam (14).

8. The calibration plane (30) is - the surface or plane of the raw material powder layer applied on the carrier (102); - the surface of the burn-off film applied on the carrier (102), and - the surface of the calibration plate (28), and / or The calibration pattern (p １，１ , p ２，１ , p １，２ , p ２，２ , p １，ｊ , p ２，ｊ , p ｉ，ｊ )teeth, - the calibration pattern (p １，１ , p ２，１ , p １，２ , p ２，２ , p １，ｊ , p ２，ｊ , p ｉ，ｊ ) is projected onto the calibration plane (30) and / or while the calibration pattern (p １，１ , p ２，１ , p １，２ , p ２，２ , p １，ｊ , p ２，ｊ , p ｉ，ｊ After the irradiation of the calibration pattern (p) onto the calibration plane is completed, １，１ , p ２，１ , p １，２ , p ２，２ , p １，ｊ , p ２，ｊ , p ｉ，ｊ an optical detection device (24) for recording said illumination of the a photosensitive sensor arrangement arranged on or below said calibration plane (30), The method according to any one of claims 1 to 7, wherein the detection is performed by at least one of the following:

9. The calibration pattern (p １，１ , p ２，１ , p １，２ , p ２，２ , p １，ｊ , p ２，ｊ , p ｉ，ｊ ) includes at least one straight line, and the focal position of the radiation beam (14) in the z direction perpendicular to the calibration plane (30) is determined in step (v) by the calibration pattern (p １，１ , p ２，１ , p １，２ , p ２，２ , p １，ｊ , p ２，ｊ , p ｉ，ｊ 9. The method according to claim 1 , wherein the width of the straight line is determined based on an evaluation of the width of the straight line of the first line.

10. The calibration pattern (p ｉ，ｊ ｏ，││ , p ｉ，ｊ ｏ，⊥ , p ｉ，ｊ Ｔｍ，││ , p ｉ，ｊ Ｔｍ，⊥ At least one of the first blocks (p) is defined by a first plurality of substantially parallel straight lines. ｉ，ｊ ｏ，││ , p ｉ，ｊ Ｔｍ，││ ) and a second block (p) defined by a second plurality of substantially parallel straight lines. ｉ，ｊ ｏ，⊥ , p ｉ，ｊ Ｔｍ，⊥ ), and the second block (p ｉ，ｊ ｏ，⊥ , p ｉ，ｊ Ｔｍ，⊥ The second plurality of substantially parallel straight lines of the first block (p ｉ，ｊ ｏ，││ , p ｉ，ｊ Ｔｍ，││ ) is disposed substantially perpendicular to the first plurality of substantially parallel straight lines of the first block (p ｉ，ｊ ｏ，││ , p ｉ，ｊ Ｔｍ，││ 10. The method according to claim 1, wherein the first plurality of substantially parallel straight lines of the illumination system (10) extend in a radial direction, in particular with respect to an optical center of the illumination system (10).

11. Individual calibration patterns (p ｉ，ｊ , p ｉ，ｊ ｏ，││ , p ｉ，ｊ ｏ，⊥ , p ｉ，ｊ Ｔｍ，││ , p ｉ，ｊ Ｔｍ，⊥ ) or multiple calibration patterns (p ｉ，ｊ , p ｉ，ｊ ｏ，││ , p ｉ，ｊ ｏ，⊥ , p ｉ，ｊ Ｔｍ，││ , p ｉ，ｊ Ｔｍ，⊥ ) in particular in a common xy region (a １ , a ２ , a ｉ A plurality of calibration patterns (p ｉ，ｊ , p ｉ，ｊ ｏ，││ , p ｉ，ｊ ｏ，⊥ , p ｉ，ｊ Ｔｍ，││ , p ｉ，ｊ Ｔｍ，⊥ ) is the individual calibration pattern (p ｉ，ｊ , p ｉ，ｊ ｏ，││ , p ｉ，ｊ ｏ，⊥ , p ｉ，ｊ Ｔｍ，││ , p ｉ，ｊ Ｔｍ，⊥ ) or the plurality of calibration patterns (p ｉ，ｊ , p ｉ，ｊ ｏ，││ , p ｉ，ｊ ｏ，⊥ , p ｉ，ｊ Ｔｍ，││ , p ｉ，ｊ Ｔｍ，⊥ 11. The method of claim 1, further comprising providing an identification marker (32) indicating a position of said calibration plane (30) of said first calibration element (10).

12. The xy area (a １ , a ２ , a ｉ ) in the common xy region (a １ , a ２ , a ｉ ) are irradiated with a plurality of calibration patterns (p １，１ , p ２，１ , p １，２ , p ２，２ , p １，ｊ , p ２，ｊ , p ｉ，ｊ 11. The method according to claim 1, wherein the illumination position of the calibration plane (30) is determined based on an xy shift of an incidence point of the radiation beam (14) on the calibration plane (30) caused by a change in the distance in the z direction between the calibration plane (30) and the optical unit (16) of the illumination system (10).

13. A device for calibrating an irradiation system (10) for use in an apparatus (100) for producing a three-dimensional workpiece by irradiating a layer of raw material powder with a radiation beam (14) emitted by the irradiation system (10), the device comprising a control unit (26), the control unit (26) comprising: (i) determining a distance between a calibration plane (30) and an optical unit (16) of the illumination system (10) in a z-direction perpendicular to the calibration plane (30) as a first distance (z １ ) and (ii) changing the distance between the calibration plane (30) and the optical unit (16) to the first distance (z １ ), a first calibration pattern (p) is scanned using a scanner mirror (22) of the optical unit (16) disposed at a first basic angular position. １，１ ) in a first xy region (a １ ) and a second calibration pattern (p ２，１ ) in a second xy region (a ２ ), wherein in the second base angular position, the scanner mirror is pivoted by at least ±1 degree relative to the first base angular position; (iii) determining the distance between the calibration plane (30) and the optical unit (16) of the illumination system (10) in the z direction perpendicular to the calibration plane (30) as the first distance (z １ ) different from the second distance (z ２ ) and (iv) changing the distance between the calibration plane (30) and the optical unit (16) to the second distance (z ２ ), a third calibration pattern (p １，２ ) in the first xy region (a １ ), and a fourth calibration pattern (p) is scanned using the scanner mirror (22) of the optical unit (16) that is disposed at the second basic angular position in which the scanner mirror (22) is pivoted by at least ±1 degree with respect to the first basic angular position. ２，２ ) in the second xy region (a ２ controlling the illumination system (10) to illuminate the (v) the first calibration pattern (p １，１ ), the second calibration pattern (p ２，１ ), the third calibration pattern (p １，２ ), and the fourth calibration pattern (p ２，２ ) to determine a focal position of the radiation beam (14) in the z direction perpendicular to the calibration plane (30) as a function of x, y coordinates in the calibration plane (30); and (vi) a device configured to calibrate the illumination system (10) based on the determined focal position of the radiation beam (14).

14. - said control unit (26) is configured to perform steps (ii) and (iv) to (vi) for a plurality of radiation beams (14); and / or The device according to claim 13, wherein the control unit (26) is configured to carry out steps (ii) and (iv) to (vi) using the irradiation system (10) having a plurality of different temperatures (Tm).

15. In order to vary the temperature of the irradiation system (10), the control unit (26) controls at least one heating zone (h) prior to carrying out steps (ii) and (iv) to (vi). ｍ ) with heating pattern (p ｈｍ 15. The device according to claim 14, configured to heat the irradiation system (10) by irradiating a light source (12) having a wavelength of 100 nm or more.

16. The control unit (26) controls a plurality of heating patterns (p ｈｍ 15. The device according to claim 13 or 14, configured to set the illumination system (10) to a plurality of different temperatures (Tm) by irradiating the target object (10) with a plurality of different output powers of the radiation beam (14) emitted by the illumination system (10) and / or for a plurality of different irradiation times.

17. The control unit (26) - positioning the calibration plane (30) at a number of further distances (z ｆ ) from each other, and ｆ ) are each the first distance (z １ ) and the second distance (z ２ ) and - a number of further calibration patterns (p １，ｊ , p ２，ｊ ) in the first xy region (a １ ) and the second xy region (a ２ The illumination system (10) is configured to control the illumination system (10) to illuminate the first xy region (a １ ) and the second xy region (a ２ ) are respectively irradiated with the further calibration patterns (p １，ｊ , p ２，ｊ ) is determined in particular by the further distance (z ｆ 17. The device according to claim 13, wherein the number of

18. The control unit (26) controls the distance between the calibration plane (30) and the optical unit (16) to be equal to or smaller than the first distance (z １ ), the second distance (z ２ ), and a number of said further distances (z ｆ ) while being maintained at at least one of the calibration patterns (p ｉ，ｊ ) in the first xy region (a １ ) and the second xy region (a ２ ) in the calibration plane (30) different from the additional xy region (a ｉ 18. The device according to claim 13, configured to control the illumination system (10) to illuminate a light source (12) on a surface of the light source (10).

19. The control unit (26) the scanner mirror (22) in the second basic angular position is pivoted relative to the first basic angular position by at least ±2 degrees, in particular by at least ±5 degrees, preferably by at least ±10 degrees and more preferably by at least ±15 degrees; and / or - the first xy region (a １ ) and the center of the second xy region (a ２ ) corresponds to at least 15 times, in particular at least 100 times, preferably at least 500 times, and more preferably at least 1000 times the diameter of the focused radiation beam (14), 19. The device according to any one of claims 13 to 18, configured to control the illumination system (10).

20. The calibration plane (30) is - the surface or plane of the raw material powder layer applied on the carrier (102); - the surface of the burn-off film applied on the carrier (102), and the surface of the calibration plate (28), and / or The device detects the calibration pattern (p １，１ , p ２，１ , p １，２ , p ２，２ , p １，ｊ , p ２，ｊ , p ｉ，ｊ ) to detect - the calibration pattern (p １，１ , p ２，１ , p １，２ , p ２，２ , p １，ｊ , p ２，ｊ , p ｉ，ｊ ) is projected onto the calibration plane (30) and / or while the calibration pattern (p １，１ , p ２，１ , p １，２ , p ２，２ , p １，ｊ , p ２，ｊ , p ｉ，ｊ After the irradiation of the calibration pattern (p) onto the calibration plane (30) is completed, １，１ , p ２，１ , p １，２ , p ２，２ , p １，ｊ , p ２，ｊ , p ｉ，ｊ an optical detection device (24) configured to record said illumination of the light source (10) at its location; a photosensitive sensor arrangement arranged on or below said calibration plane (30), 20. The device according to claim 13, further comprising at least one detection system selected from the group consisting of:

21. The calibration pattern (p １，１ , p ２，１ , p １，２ , p ２，２ , p １，ｊ , p ２，ｊ , p ｉ，ｊ ) includes at least one line, curve, point or circle, and the control unit (26) adjusts the focal position of the radiation beam (14) in the z direction perpendicular to the calibration plane (30) in step (v) relative to the calibration pattern (p １，１ , p ２，１ , p １，２ , p ２，２ , p １，ｊ , p ２，ｊ , p ｉ，ｊ 21. The device of claim 13, configured to determine based on an evaluation of a width of the line, the point, or the circle.

22. The calibration pattern (p ｉ，ｊ ｏ，││ , p ｉ，ｊ ｏ，⊥ , p ｉ，ｊ Ｔｍ，││ , p ｉ，ｊ Ｔｍ，⊥ At least one of the first blocks (p) is defined by a first plurality of substantially parallel straight lines. ｉ，ｊ ｏ，││ , p ｉ，ｊ Ｔｍ，││ ) and a second block (p) defined by a second plurality of substantially parallel straight lines. ｉ，ｊ ｏ，⊥ , p ｉ，ｊ Ｔｍ，⊥ ), and the second block (p ｉ，ｊ ｏ，⊥ , p ｉ，ｊ Ｔｍ，⊥ The second plurality of substantially parallel straight lines of the first block (p ｉ，ｊ ｏ，││ , p ｉ，ｊ Ｔｍ，││ ) is disposed substantially perpendicular to the first plurality of substantially parallel straight lines of the first block (p ｉ，ｊ ｏ，││ , p ｉ，ｊ Ｔｍ，││ 22. The device according to claim 13, wherein the first plurality of substantially parallel straight lines of the illumination system (10) extend in a radial direction, in particular with respect to an optical center of the illumination system (10).

23. The control unit (26) is a separate calibration pattern (p ｉ，ｊ , p ｉ，ｊ ｏ，││ , p ｉ，ｊ ｏ，⊥ , p ｉ，ｊ Ｔｍ，││ , p ｉ，ｊ Ｔｍ，⊥ ) or multiple calibration patterns (p ｉ，ｊ , p ｉ，ｊ ｏ，││ , p ｉ，ｊ ｏ，⊥ , p ｉ，ｊ Ｔｍ，││ , p ｉ，ｊ Ｔｍ，⊥ ) in particular in a common xy region (a １ , a ２ , a ｉ A plurality of calibration patterns (p ｉ，ｊ , p ｉ，ｊ ｏ，││ , p ｉ，ｊ ｏ，⊥ , p ｉ，ｊ Ｔｍ，││ , p ｉ，ｊ Ｔｍ，⊥ ) is the individual calibration pattern (p ｉ，ｊ , p ｉ，ｊ ｏ，││ , p ｉ，ｊ ｏ，⊥ , p ｉ，ｊ Ｔｍ，││ , p ｉ，ｊ Ｔｍ，⊥ ) or the plurality of calibration patterns (p ｉ，ｊ , p ｉ，ｊ ｏ，││ , p ｉ，ｊ ｏ，⊥ , p ｉ，ｊ Ｔｍ，││ , p ｉ，ｊ Ｔｍ，⊥ 23. The device of claim 13, configured to control the illumination system (10) to provide an identification marker (32) indicating a position of a calibration plane (30) of a calibration target (12).

24. The control unit (26) １ , a ２ , a ｉ ) in the common xy region (a １ , a ２ , a ｉ ) are irradiated with a plurality of calibration patterns (p １，１ , p ２，１ , p １，２ , p ２，２ , p １，ｊ , p ２，ｊ , p ｉ，ｊ 24. The device according to claim 13, configured to determine an illumination position of a calibration plane (30) based on an x-y shift of an incidence point of the radiation beam (14) on the calibration plane (30) caused by a change in the distance in the z-direction between the calibration plane (30) and the optical unit (16) of the illumination system (10).

25. A computer program product comprising program portions for implementing the method according to any one of claims 1 to 12 when said computer program product is executed on one or more computing devices.

26. An apparatus (100) for producing a three-dimensional workpiece by irradiating a layer of raw material powder with a radiation beam (14), the apparatus (100) comprising an irradiation system (10) and a device according to any one of claims 13 to 24 and / or a computer readable recording medium on which a computer program product according to claim 25 is stored.

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