Method and apparatus for calibrating an optical system - Patents.com

Abstract

The present invention relates to a method and an apparatus for calibrating an optical system. A method for calibrating an optical system (24), in particular for the use of an apparatus (100) for producing a three-dimensional workpiece by irradiating a layer of raw material powder, is described. The method comprises a step (i) of generating a calibration spot (C) by irradiating a target (32) at a known position in a scanner coordinate system of a scanner (22) configured to scan the radiation beam (14) over an irradiation plane (I) with a radiation beam (14) emitted by an optical unit (16). In step (ii), a calibration beam (36) is emitted from the calibration spot (C) in the direction of the optical system (24) to be calibrated. In step (iii), the optical system (24) is calibrated such that the beam path of the calibration beam (36) emitted from the calibration spot (C) is collinear with the beam path of the radiation beam (14) used in step (i) for generating the calibration spot.

Description

[Technical field]

[0001] The present invention relates to a method and apparatus for calibrating an optical system, in particular for use in an apparatus for producing three-dimensional workpieces by irradiating a layer of raw material powder with a beam of radiation.

[0002] Powder bed fusion is an additive manufacturing 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 depending on 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 successively applied to the layer already subjected to laser treatment on the carrier until the workpiece has 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] A melt pool monitoring system may be employed to monitor the irradiation process, and in particular to monitor the melting and sintering conditions at the irradiation spot where the radiation beam is incident on the raw material powder layer to generate the melt pool. The melt pool monitoring system may comprise, for example, a camera and / or a pyrometric detection unit, comprising a photodetector sensitive to radiation in a wavelength range corresponding to the thermal radiation emitted at the irradiation spot. The melt pool monitoring system thus outputs images and / or sensor values ​​indicative of, for example, the size and shape of the melt pool and / or the temperature of the melt pool. The measurement data collected by the melt pool monitoring system may be used for quality control purposes, either during the irradiation process or after completion of the build process to generate the three-dimensional workpiece.

[0004] A method and a device for calibrating a pyrometric detection device with a pyrometric detection unit configured to receive thermal radiation emitted from different points of a detection plane are described in US Pat. No. 5,399,433. The known method comprises the use of a planar substrate, a plurality of light guides and a light source. Light emitted by the light source is coupled into a first end of the light guide. A second end of the light guide is fixed to the substrate, for example to define a matrix arrangement and to emit light in a main light detection direction corresponding to the light detection direction of the pyrometric detection unit.

[0005] To calibrate the pyrometry detection unit, the substrate is positioned such that the second end of the light guide is located in a detection plane of the pyrometry detection unit. The light detection unit of the pyrometry detection unit is controlled such that light emitted from one predetermined light guide located at a predetermined position in the detection plane is detected by the pyrometry detection unit at a predetermined time. Due to the angular and / or positional dependency, the light intensity values ​​measured by the pyrometry detection unit may be different even if the intensity of light emitted from each of the multiple light guides is substantially the same. By comparing the different measurements, the pyrometry detection device can be calibrated and the angular and / or positional dependency can be compensated for. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] European Patent No. 3023747 Summary of the Invention

[0007] The present invention is directed to the object of providing a method and apparatus that allows simple and reliable calibration of an optical system that is particularly suitable for use in an apparatus for producing three-dimensional workpieces by irradiating a radiation beam onto a layer of raw material powder.

[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 optical system, particularly suitable for use in an apparatus for producing three-dimensional workpieces by irradiating a layer of raw material powder, comprising the step (i) of generating a calibration spot by irradiating a radiation beam emitted by an optical unit onto a target at a known position in a scanner coordinate system of a scanner configured to scan the radiation beam across an irradiation plane. This radiation beam used to generate the calibration spot may be an electromagnetic radiation beam or a particle radiation beam.

[0010] The illumination system of the device for producing three-dimensional workpieces may comprise a single optical unit, so that the illumination system is designed in the form of a single-beam illumination system generating a single radiation beam. However, it is also conceivable that the illumination system is designed in the form of a multiple-beam illumination system, which comprises a plurality of optical units and is thus configured to generate a plurality of radiation beams. In the latter example, the calibration spot may be generated by a selected one of the radiation beams emitted by a selected one of the optical units of the illumination system. The illumination system may further comprise a radiation source, in particular a laser source such as a diode-pumped ytterbium fiber laser. The illumination system may comprise a single radiation source. However, it is also conceivable that the illumination system comprises a plurality of radiation sources, in particular when the illumination system is designed in the form of a multiple-beam illumination system.

[0011] A scanner used to scan the radiation beam across the illumination plane may be a component of the illumination system. The scanner may comprise a pivotable scanner mirror adapted to scan the radiation beam across the illumination plane. Besides the scanner, the illumination system may further comprise a beam expander for expanding the radiation beam emitted by the radiation source and an objective lens, in particular an f-theta objective lens. Alternatively, the illumination system may comprise a beam expander including a focusing optical element.

[0012] In the context of this application, the term "scanner coordinate system" defines a coordinate system that serves to establish a correlation between the setup of the scanner, in particular the angular position of the scanner mirror, and the illumination spot, i.e. the point of incidence of the radiation beam in the illumination plane. Typically, this scanner coordinate system does not coincide with the global machine coordinate system of an apparatus for producing three-dimensional workpieces or the like. However, for the purposes of the calibration described herein, it is sufficient to define the position of the calibration spot in the illumination plane with reference to said scanner coordinate system, i.e. it is not necessary to localize the calibration spot in the global machine coordinate system.

[0013] In step (ii) of the method for calibrating an optical system, a calibration beam is emitted from a calibration spot in the direction of the optical system to be calibrated. Preferably, the radiation beam used to generate the calibration spot and this calibration beam are not emitted simultaneously. In particular, steps (i) and (ii) can be performed sequentially. Step (i) can be performed first to generate the calibration spot and step (ii) can be performed only thereafter, i.e. after completion of step (i). In particular, the calibration beam emitted from the calibration spot is directed to the optical system via a scanner used in step (i) to direct the radiation beam to the calibration spot. For example, a scanner mirror can be maintained in the same angular position for generating the calibration spot by irradiating a target with a radiation beam in step (i) and for directing the calibration beam emitted from the calibration spot in step (ii) to the optical system to be calibrated.

[0014] In step (iii), the optical system is calibrated such that the beam path of the calibration beam emitted from the calibration spot is collinear with the beam path of the radiation beam used in step (i) to generate the calibration spot. To calibrate the optical system, at least one optical component of the optical system, for example an optical mirror, may be adjusted by a suitable adjustment device, for example an actuator that can be manually or automatically operated, such that the optical centre of the optical system is aligned with the calibration spot.

[0015] The method described herein for calibrating an optical system is suitable for calibrating all optical systems designed in the form of an in-line system with respect to the beam path of the radiation beam, i.e. all optical systems emitting or receiving radiation following the same beam path as the radiation beam. With this method it is possible to omit the exact positioning of the calibration beam emitting device in the machine coordinate system. Furthermore, the method does not depend on the scan field correction of the optical system to be calibrated and on the accuracy of the calibration plates used so far. As a result, the method allows a reliable calibration of the optical system in a short time, which is more or less independent of the operator's skill. The alignment of the optical system can be performed either manually or with the aid of software running on a control unit which controls the corresponding actuators of the optical system.

[0016] In step (i), the target may be positioned in the illumination plane in a region expected to include the known position in the scanner coordinate system of the scanner. The illumination plane irradiated with the radiation beam in the calibration method described herein may coincide with the illumination plane on which the radiation beam is incident during normal operation of the apparatus for producing the three-dimensional workpiece. For example, in the present calibration method, the illumination plane irradiated with the radiation beam may coincide with the illumination plane defined by the surface of the raw material powder layer that is selectively irradiated during operation of the apparatus for producing the three-dimensional workpiece. The region for positioning the target may be selected taking into account typical positional deviations or tolerances that occur when scanning the radiation beam across the illumination plane. However, it is also conceivable to use a target sized and dimensioned to cover the entire surface area of ​​the illumination plane.

[0017] The calibration spot generated in step (i) may be a visible spot on a target, on which a calibration beam emitting device can be placed before the calibration beam is emitted in the direction of the optical system to be calibrated. Alternatively, the calibration spot generated in step (i) may be defined by a pinhole generated by irradiating the target with the radiation beam, the calibration beam emitting device being in place already during the generation of the calibration spot. The pinhole generated by irradiating the target with the radiation beam may be a through hole extending through the target. However, it is also conceivable that the irradiation of the target with the radiation beam merely generates a calibration spot that transmits the light emitted by the calibration beam emitting device.

[0018] However, the calibration spots may take other forms or be generated in a different manner, for example the calibration spots may be defined by beam-reflecting and / or beam-scattering structures that are generated in the target by irradiating the target with a beam of radiation.

[0019] The target may be a film, in particular an aluminum film or a transparent film, and may be arranged in the illumination plane, for example the target may be fixed in a suitable clamping device and extend over at least a region of the illumination plane.

[0020] The calibration beam may be generated by a light source and may be reflected and / or scattered from a calibration spot.

[0021] In particular, when the calibration beam emitting device comprises a light source for generating the calibration beam, the target is preferably arranged in the beam path of the calibration beam generated by the light source, between the light source and the optical system to be calibrated.

[0022] Furthermore, in a preferred embodiment of the method for calibrating an optical system, a shutter may be arranged in the beam path of the radiation beam between the target and the light source, at least during the generation of the calibration spot in step (i). Arranging a shutter between the target and the light source helps to protect the light source from interaction with the radiation beam when generating the calibration spot. However, this shutter may be omitted, for example, if the light source is designed to be insensitive to the radiation beam or if the light source is only placed in the beam path of the radiation beam after the formation of the pinhole. Also, a shutter is not mandatory in case the calibration beam is a beam that is reflected and / or scattered by a beam reflecting or beam scattering structure generated in the target.

[0023] As already alluded to above, the optical system to be calibrated is preferably a system designed in the form of an in-line system with respect to the beam path of the radiation beam, emitting or receiving radiation following the same beam path as the radiation beam. For example, the optical system may include an optical sensor system of a melt pool monitoring system, a camera-based system, a photodiode-based system, and / or an optical coherence tomography system.

[0024] The method of the present disclosure may further comprise a step (iv) of generating a further calibration spot by irradiating a further radiation beam emitted by a further optical unit onto the target at a known position within a further scanner coordinate system of a further scanner configured to scan the further radiation beam across the irradiation plane.

[0025] A further calibration beam may be emitted from the further calibration spot in the direction of the further optical system to be calibrated. The further optical system to be calibrated may be associated with a further optical unit generating a further radiation beam. The further optical system may be calibrated such that the beam path of the further calibration beam emitted from the further calibration spot is collinear with the beam path of the further radiation beam used in step (iv) generating the further calibration spot. In this way, the further optical system may be calibrated in a similar manner to the optical system described above.

[0026] However, alternatively or additionally, it is also conceivable to emit a further calibration beam from a further calibration spot in the direction of the optical system. The scanner coordinate system of the scanner and / or the further scanner coordinate system of the further scanner can then be adjusted to coincide. In this way, the calibration method can also be used to collinearly align multiple optical units.

[0027] The present disclosure also relates to an apparatus for calibrating an optical system, and in particular to an apparatus for use in an apparatus for producing a three-dimensional workpiece by irradiating a raw material powder layer. The apparatus includes a target, which is at a known position in a scanner coordinate system of a scanner configured to scan the radiation beam over an irradiation plane to generate a calibration spot, and which is configured to be irradiated with a radiation beam emitted by an optical unit. The apparatus further comprises a calibration beam emission device configured to generate a calibration beam emitted from the calibration spot. Furthermore, the apparatus comprises an adjustment device configured to enable the optical system to be calibrated such that the beam path of the calibration beam emitted from the calibration spot is collinear with the beam path of the radiation beam used to generate the calibration spot. For example, the adjustment device may comprise an actuator operable manually or automatically, such that either manual or automatic calibration of the optical system is possible.

[0028] The control unit may be provided in an apparatus for calibrating an optical system, where the optical system is configured to control the optical unit and the calibration beam emitting device such that the radiation beam used to generate the calibration spot and said calibration beam are not emitted simultaneously. The control unit may be configured to manually or automatically control the emission of the radiation beam and the calibration beam. In particular, the control unit may be configured to control the optical unit and the calibration beam emitting device such that the radiation beam used to generate the calibration spot is emitted first and the calibration beam is only emitted after the emission of the radiation beam has finished.

[0029] The apparatus may further comprise a positioning device configured to position the target in the illumination plane within a region expected to contain the known position in a scanner coordinate system of the scanner.

[0030] The calibration spot may be defined by a pinhole created by shining a beam of radiation onto the target, or alternatively, the calibration spot may be defined by beam-reflecting or beam-scattering structures created in the target by shining a beam of radiation onto the target.

[0031] The target may be a film, in particular an aluminium film or a transparent film, adapted to be placed in the illumination plane.

[0032] The calibration beam emitting device may comprise a light source or a beam reflecting or scattering structure created in the target by irradiating the target with a beam of radiation.

[0033] The target may be positioned in the beam path of the calibration beam generated by the light source, between the light source and the optical system to be calibrated.

[0034] The apparatus may further comprise a shutter configured to be positioned in the beam path of the radiation beam between the target and the light source, at least during generation of the calibration spot.

[0035] The optical system to be calibrated may include an optical sensor system, particularly a melt pool monitoring system, a camera-based system, a photodiode-based system, and / or an optical coherence tomography system. The optical system may also include additional components such as shutters, lenses, and / or mirrors.

[0036] To generate a further calibration spot, the apparatus may comprise a further optical unit configured to irradiate a further radiation beam onto a target at the known position within a further scanner coordinate system of a further scanner configured to scan the further radiation beam across the irradiation plane.

[0037] The calibration beam emitting device may be configured to emit a further calibration beam from the further calibration spot in the direction of the further optical system to be calibrated, and furthermore the device may comprise a further adjusting device, which allows to calibrate the further optical system such that the beam path of the further calibration beam emitted from the further calibration spot is collinear with the beam path of the further radiation beam used to generate the further calibration spot.

[0038] Alternatively or additionally, the calibration beam emitting device may be configured to emit a further calibration beam from a further calibration spot in the direction of said optical system. The adjustment device and / or the further adjustment device may be configured to coincide a scanner coordinate system of the scanner and / or a further scanner coordinate system of the further scanner.

[0039] Preferred embodiments of the invention will now be described in more detail with reference to the accompanying schematic drawings. [Brief description of the drawings]

[0040] [Figure 1] FIG. 1 shows an apparatus for producing a three-dimensional workpiece by irradiating a layer of raw material powder with a beam of radiation.

[0041] [Diagram 2] FIG. 2 illustrates the steps of generating a calibration spot in a method for calibrating the optical system employed in the apparatus depicted in FIG.

[0042] [Diagram 3] FIG. 3 illustrates the step of emitting a calibration beam generated by a light source from a calibration spot in the direction of an optical system to be calibrated.

[0043] [Figure 4] FIG. 4 shows a schematic diagram of an apparatus for calibrating an optical system such as that shown in FIGS. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0044] FIG. 1 shows an apparatus 100 for producing three-dimensional workpieces by an AM process. The apparatus 100 comprises a carrier 102 and a powder applicator 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, it is preferred to process the powder to a particle size of less than 100 μm. The carrier 102 and the powder applicator 104 are housed 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 12. The carrier 102 may be equipped with a heater or cooler.

[0045] The apparatus 100 further comprises an irradiation system 10 for selectively irradiating laser radiation onto the raw material powder layer applied on the carrier 102. In the embodiment of the apparatus 100 shown in Fig. 1, the irradiation system 10 comprises a radiation beam source 12 configured to generate a radiation beam 14 and a further radiation beam source 12' configured to generate a further radiation beam 14'. The radiation beam source 12, 12' may be a laser beam source configured to generate a laser beam. An optical unit 16 for guiding and processing the radiation beam 14 generated by the radiation beam source 12 is associated with the radiation beam source 12. A further optical unit 16' for guiding and processing the further radiation beam 14' generated by the further radiation beam source 12' is associated with the further radiation beam source 12'. However, it is also conceivable that the optical unit 16 and the further optical unit 16' are associated with a single radiation beam source and / or that the irradiation system 10 comprises multiple optical units and is configured to generate more than two radiation beams.

[0046] Each of the optical units 16, 16' may comprise two lenses (not shown), both having positive refractive power. One lens may be configured to collimate the laser light emitted by the radiation beam source 12, 12', such that a collimated or substantially collimated radiation beam is generated. A further lens may be configured to focus the collimated (or substantially collimated) radiation beam 14, 14' at a desired position in the z-direction. Each of the optical units 16 further comprises a pivotable scanner mirror 22, 22', which serves to deflect the radiation beam 14, 14' and thus scan the radiation beam 14, 14' in the x-direction and the y-direction over an illumination plane I, typically defined by the surface of a raw material powder layer applied on the carrier 102 to be selectively illuminated, during operation of the apparatus 100.

[0047] 1 in the form of an optical sensor system, forms part of the melt pool monitoring system and thus serves to observe the melt pool generated when the radiation beam 14, 14' impinges on the raw material powder. In particular, each of the optical systems 24, 24' outputs a sensor value related to the temperature of the melt pool or a melt pool temperature indication.

[0048] Each of the optical systems 24, 24' is designed in the form of an in-line system receiving thermal radiation 26, 26' following the same beam path as the radiation beam 14, 14'. For this purpose, each of the optical units 16, 16' comprises a semi-transparent beam splitter 28, 28', which is suitable for passing the radiation beam 14, 14' emitted by the radiation source 12, 12' but deflecting the thermal radiation beam 26, 26' emitted from the melt pool. Thus, the thermal radiation beam 26, 26' emitted from the melt pool, after being deflected by the scanner mirror 22, 22', is finally directed by the beam splitter 28, 28' to the optical system 24, 24'.

[0049] The control unit 30 may be provided either for controlling only the operation of the optical systems 24, 24' or may also be provided for controlling further components of the apparatus 100, such as the irradiation system 10 and / or the powder application device 104.

[0050] A controlled gas atmosphere, preferably an inert gas atmosphere, is established in the process chamber 106 by supplying a shielding gas to the process chamber 106 via a process gas inlet 112. The gas is directed through the process chamber 106 and across the layer of raw material powder applied on the carrier 102 before exiting the process chamber 106 via a process gas outlet 114. The process gas is recirculated from the process gas outlet 114 back to the process gas inlet 112 where it may be cooled or heated.

[0051] During operation of the apparatus 100 for producing three-dimensional workpieces, 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 over the carrier 102, e.g. under the control of the control unit 30. The raw material powder layer is then selectively irradiated according to the geometry of the corresponding layer of the workpiece 110 to be produced by the irradiation device 10, e.g. again under the control of the control unit 30. The steps 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 are repeated until the workpiece 110 reaches the desired shape and size.

[0052] A method for calibrating the optical system 24 to enable accurate detection of the melt pool radiation / temperature independent of the melt pool's position in the illumination plane I is described with reference to FIGS.

[0053] As shown in Fig. 2, the target 32 ​​is placed in the illumination plane I. In the embodiment shown therein, the target 32 ​​is designed in the form of a thin film, in particular an aluminium film. In step (i) of the method for calibrating the optical system 24, the radiation beam 14 emitted by the illumination system 10 of the device 100 is irradiated onto the target 32, thereby generating a calibration spot C at a known position in the scanner coordinate system of the scanner 22, which scans the radiation beam 14 across the illumination plane I. Thus, the position of the calibration spot C in the xy illumination plane I may not be known exactly in terms of its xy coordinates in the machine coordinate system, but it can still be determined by its scanner coordinates. However, the target 32 ​​is positioned in the scanner coordinate system of the scanner 22 in a region of the illumination plane I that is expected to include said known position. In particular, the radiation beam 14 incident on the target 32 ​​generates a calibration spot C in the shape of a pinhole extending through the target 32.

[0054] A calibration beam emitting device designed in the form of a light source 34 capable of generating a calibration beam 36 is shown below the target 32. In particular, the light source 34 is positioned, see Fig. 3, in such a position relative to the calibration spot C that the calibration beam 36 generated by the light source 34 is emitted from the calibration spot C in step (ii) of the method for calibrating the optical system 24 and is directed to the optical system 24 via the scanner mirror 22 and the semi-transparent beam splitter 28 used in step (i) to direct the radiation beam 14 to the calibration spot C. Steps (i) and (ii) are performed sequentially, i.e. the calibration beam 36 is emitted from the calibration spot C only after the emission of the radiation beam 14 has been stopped after the generation of the calibration spot C has been completed. The scanner mirror 22 can be maintained at the same angular position to generate a calibration spot C by irradiating the radiation beam 14 onto the target 32 ​​in step (i), and to direct the calibration beam 36 emitted from the calibration spot C towards the optical system 24 to be calibrated in step (ii).

[0055] In step (iii) of the method of calibrating the optical system 24, the optical system 24 is calibrated such that the beam path of the calibration beam 36 emitted from the calibration spot C is collinear with the beam path of the radiation beam 14 used in step (i) to generate the calibration spot C. For example, the optical system 24 may be calibrated by adjusting the optical mirrors of the optical system 24 under the control of the control unit 30 such that the optical centre of the optical system 24 is aligned with the calibration spot C indicated by the arrow in Fig. 3. However, it is also conceivable that the calibration of the optical system 24 comprises adjusting by moving or tilting the sensor of the optical system 24, twisting the plane plate (beam offset), or other suitable adjustment procedures.

[0056] In the embodiment shown in the figures, the target 32 ​​is arranged in the beam path of the calibration beam 36 generated by the light source 34, between the light source 34 and the optical system 24 to be calibrated. Furthermore, the target 32 ​​is arranged in the beam path of the radiation beam 14, between the radiation source 12 and the light source 34. In order to protect the light source 34 from damage that may be caused by the radiation beam 14 when generating the pinhole, a shutter 38 is arranged in the beam path of the radiation beam 14, between the target 32 ​​and the light source 34, at least during calibrating the calibration spot C in step (i), although in step (ii) the shutter 38 is removed to allow unhindered propagation of the calibration beam 36 through the pinhole generated in the target 32.

[0057] However, the shutter 38 may be omitted, for example, if the light source 34 is designed to be insensitive to the radiation beam 14 or if the light source 34 is only placed in the beam path of the radiation beam 14 after the formation of the pinhole. The shutter 38 is also not mandatory if the calibration beam 36 is not generated by the light source 34 but instead is a beam that is reflected and / or scattered by a calibration beam emitting device designed in the form of a beam reflecting or beam scattering structure generated in the target in step (i).

[0058] The further optical system 24' may be configured in the same way as the optical system 24. In particular, in step (iv) a further calibration spot may be generated by irradiating the further radiation beam 14' emitted by the further optical unit 16' onto a target 32 ​​at a known position in a further scanner coordinate system of a further scanner 22' configured to scan the further radiation beam 14' across an illumination plane I. The further calibration beam may be emitted from the further calibration spot in the direction of the further optical system 24' to be calibrated.

[0059] The further optical system 24' may then be calibrated such that the beam path of the further calibration beam emitted from the further calibration spot is collinear with the beam path of the further radiation beam 14' used in step (iv) for generating the further calibration spot. Similar to the optical system 24, the further optical system 24' may also be calibrated by adjusting the optical mirrors of the further optical system 24 under the control of the control unit 30 such that the optical centre of the further optical system 24' is aligned with the further calibration spot. However, it is also conceivable that the calibration of the further optical system 24' may comprise adjusting by moving or tilting a sensor of the further optical system 24', twisting a plane plate (beam offset) or any other suitable adjustment procedure that may be performed manually at a.

[0060] Furthermore, a further calibration beam can be emitted from the further calibration spot in the direction of the optical system 24. The scanner coordinate system of the scanner 22 and / or the further scanner coordinate system of the further scanner 22' can then be adjusted to coincide. In this way, the optical units 16, 16' can be used collinearly aligned.

[0061] In this manner, the present calibration method can also be used to collinearly align multiple optical units.

[0062] An apparatus 40 for calibrating the optical system 24, 24' shown in Figures 2 and 3 and described above is shown diagrammatically in Figure 4. The apparatus 40 comprises a positioning device 42 for positioning and properly mounting the target 32 ​​at a desired position in the illumination plane I. The light source 34 is mounted on the positioning device 42 so as to be located below the target 32. The shutter 38 is movably housed in the positioning device 42. In particular, the shutter is displaceable relative to the positioning device 42, the light source 34 and the target 32 ​​as indicated by the arrow b in Figure 4.

[0063] The device 40 may also comprise further components, such as a filter arrangement (not shown) that allows for example to arrange different optical filters in the beam path of the calibration beam 36 emanating from the calibration spot C. The optical filters can vary the intensity of the calibration beam 36 emanating from the calibration spot C and can thus be used to calibrate the radiation intensity detection function of the optical unit 24. Furthermore, the device 40 may also be used to adjust the focus of the optical unit 24, for example in such a way that substantially only the calibration beam 36 emanating from the calibration spot C is directed towards the optical unit 24.

Claims

1. A method for calibrating an optical system (24), in particular for use with an apparatus (100) for producing a three-dimensional workpiece by irradiating a layer of raw material powder, said method comprising the steps of: (i) generating a calibration spot (C) by illuminating a target (32) at a known position in a scanner coordinate system of a scanner (22) configured to scan the radiation beam (14) emitted by an optical unit (16) across an illumination plane (I); (ii) emitting a calibration beam (36) from said calibration spot (C) in the direction of said optical system (24) to be calibrated; (iii) calibrating the optical system (24) such that the beam path of the calibration beam (36) emitted from the calibration spot (C) is collinear with the beam path of the radiation beam (14) used in step (i) to generate the calibration spot (C).

2. 2. The method of claim 1, wherein in step (i), the target (32) is positioned in the illumination plane (I) within a region expected to contain the known position in the scanner coordinate system of the scanner (22).

3. 3. The method of claim 1 or 2, wherein the calibration spot (C) generated in step (i) is defined by a pinhole generated by irradiating the target (32) with the radiation beam (14) or by beam reflecting structures and / or beam scattering structures generated in the target (32) by irradiating the target (32) with the radiation beam (14).

4. the target (32) is a film, in particular an aluminum film or a transparent film, and is located in the illumination plane (I); and / or The method according to any one of claims 1 to 3, wherein the calibration beam (36) is generated by a light source (34) or is reflected and / or scattered from the calibration spot (C).

5. the target (32) is arranged in the beam path of the calibration beam (36) generated by the light source (34) between the light source (34) and the optical system (24) to be calibrated; and / or 5. The method of claim 4, wherein a shutter (38) is positioned in the beam path of the radiation beam (14) between the target (32) and the light source (34) at least during generation of the calibration spot (C) in step (i).

6. 6. The method according to any one of claims 1 to 5, wherein the optical system (24) comprises an optical sensor system, in particular a melt pool monitoring system, a camera-based system, a photodiode-based system, and / or an optical coherence tomography system.

7. The method comprises: The method according to any one of claims 1 to 6, further comprising the step of (iv) generating a further calibration spot by irradiating a further radiation beam (14') emitted by a further optical unit (16') onto the target (32) at a known position in a further scanner coordinate system of a further scanner (22') configured to scan the further radiation beam (14') across the illumination plane (I).

8. The method comprises: (v) emitting a further calibration beam from said further calibration spot in the direction of a further optical system (24') to be calibrated; 8. The method of claim 7, further comprising the step of: (vi) calibrating the further optical system (24') such that a beam path of the further calibration beam emitted from the further calibration spot is collinear with a beam path of the further radiation beam (14') used to generate the further calibration spot in step (iv).

9. The method comprises: (vii) emitting a further calibration beam from said further calibration spot in the direction of said optical system (24); The method according to claim 7 or 8, further comprising the step of (viii) adjusting the scanner coordinate system of the scanner (22) and / or the further scanner coordinate system of the further scanner (22') to coincide.

10. A device for calibrating an optical system (24), in particular for use in a device (100) for producing a three-dimensional workpiece by irradiating a layer of raw material powder, said device comprising: (i) a target (32) configured to be illuminated with a radiation beam (14) emitted by an optical unit (16) to generate a calibration spot (C), the target (32) being at a known position in a scanner coordinate system of a scanner (22) configured to scan said radiation beam (14) across an illumination plane (I); (ii) a calibration beam emission device configured to emit a calibration beam (36) from said calibration spot (C) in the direction of said optical system to be calibrated; (iii) an adjustment device configured to enable calibrating the optical system (24) such that a beam path of the calibration beam (36) emitted from the calibration spot (C) is collinear with a beam path of the radiation beam (14) used to generate the calibration spot (C).

11. 11. The apparatus of claim 10, further comprising a positioning device (42) configured to position the target (32) within the illumination plane (I) within a region expected to contain the known position in the scanner coordinate system of the scanner (22).

12. The apparatus of claim 10 or 11, wherein the calibration spot (C) is defined by a pinhole created by irradiating the target (32) with the radiation beam (14) or by a beam reflecting structure and / or a beam scattering structure created in the target (32) by irradiating the target (32) with the radiation beam (14).

13. The apparatus according to any one of claims 10 to 12, wherein the target (32) is a film, in particular an aluminum film or a transparent film, configured to be arranged in the irradiation plane (I) and / or the calibration beam emitting device comprises a light source (34) or a beam reflecting structure and / or a beam scattering structure generated in the target (32) by irradiating the target (32) with the radiation beam (14).

14. 14. The apparatus of claim 13, wherein the target (32) is positioned between the light source (34) and the optical system (24) to be calibrated in the beam path of the calibration beam (36) generated by the light source (34), and / or the apparatus further comprises a shutter (38), the shutter (38) being configured to be positioned between the target (32) and the light source (34) in the beam path of the radiation beam (14) at least while the calibration spot (C) is being generated.

15. 15. The apparatus according to any one of claims 10 to 14, wherein the optical system (24) comprises an optical sensor system, in particular a melt pool monitoring system, a camera-based system, a photodiode-based system, and / or an optical coherence tomography system.

16. The apparatus comprises: The apparatus of any one of claims 10 to 15, further comprising a further optical unit (16') configured to irradiate the further radiation beam (14) onto the target (32) at a known position in a further scanner coordinate system of a further scanner (22') configured to scan the further radiation beam (14') across the illumination plane (I) to generate a further calibration spot.

17. (v) the calibration beam emitting device is configured to emit a further calibration beam from the further calibration spot in the direction of a further optical system (24') to be calibrated; and The apparatus comprises:

17. The apparatus of claim 16, further comprising: (vi) a further adjustment device configured to enable calibrating the further optical system (24') such that a beam path of the further calibration beam emitted from the further optical system is collinear with a beam path of the further radiation beam used to generate the further calibration spot.

18. (vii) the calibration beam emission device is configured to emit a further calibration beam from the further calibration spot in the direction of the optical system (24); and (viii) Apparatus according to claim 16 or 17, wherein the adjustment device and / or the further adjustment device are configured to adjust the scanner coordinate system of the scanner (22) and / or the further scanner coordinate system of the further scanner (22') to coincide.

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