Device for Additive Manufacturing of a Workpiece

Abstract

A device for additive manufacturing of a workpiece includes a production platform supporting a defined material layer of particulate material, a structuring tool, an inspection sensor, a control unit, and a position encoder. The inspection sensor has a line scan camera and a line light source and is movable along a movement direction relative to the production platform. The position encoder generates a position signal representing a respective instantaneous position of the inspection sensor relative to the production platform. The control unit generates a spatially resolved image of the defined layer using the line light source, the line scan camera, and the position signal. The control unit controls the structuring tool in order to produce a defined workpiece layer by selectively solidifying particulate material of the defined material layer based on the image of the defined material layer and/or an image of a previously produced workpiece layer.

Claims

1. A device for additive manufacturing of a workpiece, the device comprising: a production platform configured to support a defined material layer of particulate material; a structuring tool configured to selectively solidify particulate material of the defined material layer supported on the production platform in order to produce a defined workpiece layer; an inspection sensor configured to inspect at least one of the defined material layer and the defined workpiece layer; an evaluation and control unit including an interface configured to obtain a data set defining the workpiece in a plurality of successive workpiece layers one on top of another; and a position encoder, wherein: the evaluation and control unit is configured to control the structuring tool depending on the data set in order to produce the plurality of successive workpiece layers one on top of another from the particulate material, the inspection sensor is movable along a movement direction relative to the production platform, the inspection sensor includes a line scan camera and a line light source, the line scan camera and the line light source each extend transversely with respect to the movement direction, the inspection sensor is coupled to the position encoder such that the position encoder generates a position signal representing a respective instantaneous position of the inspection sensor relative to the production platform, the evaluation and control unit is configured to generate a spatially resolved image of the at least one of the defined material layer and the defined workpiece layer using the line light source, the line scan camera, and the position signal, and the evaluation and control unit is configured to control the structuring tool depending on the spatially resolved image.

2. The device of claim 1 further comprising: a layer forming tool configured to form the defined material layer of the particulate material on the production platform, wherein the layer forming tool is movable relative to the production platform along the movement direction.

3. The device of claim 2 wherein the inspection sensor is coupled to the layer forming tool.

4. The device of claim 2 wherein: the layer forming tool includes a material outlet configured to selectively discharge particulate material, the line scan camera is arranged upstream of the material outlet in the movement direction, the inspection sensor includes a second line scan camera, and the second line scan camera is arranged downstream of the material outlet in the movement direction.

5. The device of claim 1 wherein: the line light source includes a plurality of light elements, and the evaluation and control unit is configured to selectively activate or deactivate the plurality of light elements so as to generate an illumination of the at least one of the defined material layer and the workpiece layer from varying directions.

6. The device of claim 1 wherein: the line light source is arranged upstream of the line scan camera in the movement direction, the inspection sensor includes a second line light source, and the second line light source is arranged downstream of the line scan camera in the movement direction.

7. The device of claim 1 wherein the line light source has a plurality of light elements producing a plurality of different light colors.

8. The device of claim 1 wherein: the line scan camera has a first optical axis running orthogonally with respect to the defined material layer, the inspection sensor includes a second line scan camera, and the second line scan camera has a second optical axis running obliquely with respect to the defined material layer.

9. The device of claim 8 wherein the line scan camera and the second line scan camera are arranged such that the first optical axis and the second optical axis intersect at the defined material layer.

10. The device of claim 8 wherein the line scan camera is configured to generate a thermal line scan image of the at least one of the defined material layer and the defined workpiece layer.

11. The device of claim 8 wherein the evaluation and control unit is configured to correlate with one another a first image captured by the line scan camera and a second image captured by the second line scan camera.

12. The device of claim 1 wherein the line light source is oriented obliquely with respect to the at least one of the defined material layer and the defined workpiece layer.

13. The device of claim 1 wherein the line scan camera is a multi-line scan camera including a plurality of adjacent camera elements in the movement direction.

14. The device of claim 1 further comprising a light pattern generator arranged above the production platform and configured to generate a defined light pattern on the at least one of the defined material layer and the defined workpiece layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] The present disclosure will become more fully understood from the detailed description and the accompanying drawings.

[0042] FIG. 1 shows a schematic illustration of an example embodiment of the novel device.

[0043] FIG. 2 shows a schematic illustration of an example embodiment of the inspection sensor of the device in accordance with FIG. 1.

[0044] FIG. 3 shows a schematic illustration of a further example embodiment of the inspection sensor of the device in accordance with FIG. 1.

[0045] FIG. 4 shows a schematic illustration of a further example embodiment of the inspection sensor of the device in accordance with FIG. 1.

[0046] FIG. 5 shows a schematic illustration of a further example embodiment of the inspection sensor of the device in accordance with FIG. 1.

[0047] FIG. 6 shows a schematic illustration of an example arrangement of two line light sources and a multi-line scan camera for the inspection sensor of the device in accordance with FIG. 1.

[0048] FIG. 7 shows a flowchart for explaining the mode of operation of an example embodiment of the novel device.

[0049] FIG. 8 shows a further flowchart for explaining the inspection of the material surface in accordance with an example embodiment of the novel device.

[0050] In the drawings, reference numbers may be reused to identify similar and/or identical elements.

DETAILED DESCRIPTION

[0051] In FIG. 1, an example embodiment of the novel device is designated in its entirety by the reference numeral 10. The device 10 has a production platform 12, on which a workpiece 14 is additively manufactured. The workpiece 14 is built up from a material stack layer by layer from the bottom to the top in temporally successive steps. A currently topmost workpiece contour or workpiece layer is designated by the reference numeral 16. A topmost material layer from which the workpiece layer 16 is produced is designated by the reference numeral 18.

[0052] After a respective workpiece layer 16 has been completed, a new material layer 18 composed of a pulverulent material 20, for instance a metallic, plastics and/or ceramic material, is distributed on the layer stack 24 with the aid of a layer forming tool in the form of a doctor blade 22. For this purpose, the production platform 12 is typically lowered by the height of the next material layer in the direction of the arrow 26 and the particulate material 20 is taken from a reservoir 28 and distributed on the present layer stack 24 with the aid of the doctor blade 22. For this purpose, the doctor blade can be moved relative to the production platform 12 in the direction of the arrow 29. Other example embodiments can include applying a workpiece material without a doctor blade, for instance a locally selective feed of a workpiece material 20 by means of a tool head (not illustrated here) which is movable relative to the production platform 12.

[0053] Designated by the reference numeral 30, a structuring tool is illustrated in a simplified manner. In some example embodiments, the structuring tool 30 generates a laser beam 32 and moves it relative to the production platform 12 and the material layer 18 to be structured. By means of the laser beam 32, the material particles are selectively melted or totally melted, such that they solidify upon cooling.

[0054] In other example embodiments, the structuring tool 30 can generate an electron beam in order to structure a workpiece layer 16 on the production platform 12. Furthermore, in further example embodiments, as an alternative or in addition to the energy beam 30, the structuring tool 30 could apply a workpiece material locally selectively on the production platform 12 or the topmost workpiece layer 16, for example in the form of a material powder that is introduced into a melt pool In some example embodiments, the device 10 can include more than one structuring tool 30, that is to say it can use for instance two or more laser and/or electron beams for producing a workpiece layer.

[0055] The structuring tool 30, sometimes simply referred to as a writing laser below, is connected to an evaluation and control unit 34, which controls the movement of the laser beam 32 along the material surface. The evaluation and control unit 34 has an interface 36, via which a data set 38 can be read in, which defines the workpiece 14 to be manufactured in a plurality of workpiece layers arranged one on top of another. Accordingly, the evaluation and control unit 34 controls the movement of the laser beam 32 depending on the data set 38, wherein the laser beam 32 describes a respective trajectory, which results from the data set 38, in each workpiece layer 16 to be manufactured. In some example embodiments, the evaluation and control unit 34 is realized with the aid of commercially available personal computers on which a commercially available operating system, such as, for instance, Microsoft Windows, OSX or Linux, and one or more control programs 40 are implemented. In some cases, the evaluation and control unit 34 can be realized as a soft PLC on a commercially available PC. Additionally or alternatively, the evaluation and control unit 34 can be realized with the aid of dedicated control hardware in the form of a PLC and/or with the aid of one or more ASICs, FPGAs, microcontrollers, microprocessors or comparable logic circuits. The evaluation and control unit 34 can be realized with the aid of a plurality of controllers and/or personal computers which are networked or communicate with one another in some other way, for instance via a dual-ported RAM (not illustrated here).

[0056] The device 10 furthermore has an inspection sensor 42 configured to inspect the surface of the layer stack 24. The inspection sensor 42 here includes one or more line scan cameras 44 and one or more line light sources 46 (see FIGS. 2, 4, 5, 6), which are each connected to the evaluation and control unit 34. As illustrated in FIG. 1, the inspection sensor 42 here is arranged on the doctor blade 22. The doctor blade 22 here is coupled to a position encoder 48. With the aid of the position encoder 48, the evaluation and control unit 34 can determine an instantaneous position of the doctor blade 22 relative to the production platform 12. Accordingly, the position encoder 48 generates a position signal that is representative of the respective instantaneous position of the inspection sensor 42 relative to the production platform 12 and thus also relative to the layer stack 24. The evaluation and control unit 34 can record a spatially resolved image of the surface of the layer stack 24 with the aid of the line scan camera 44 and using the position signal from the position encoder 48. Accordingly, the evaluation and control unit 34 can record spatially resolved images of the respective topmost material layer 18 and/or workpiece layer 16 with the aid of the line scan camera 44 and using the position signal.

[0057] The one or more line light sources extends transversely with respect to the movement direction 29. In example embodiments, the line light sources and line scan cameras extend over the entire width of the production platform. On account of the extent of the line light sources in a transverse direction and on account of the movement relative to the layer stack 24, the evaluation and control unit 34 is able to illuminate the upper material layer 18 comprising the particulate material and/or the upper workpiece layer 16 from a plurality of different directions. By way of example, two illumination directions are indicated here by the reference numerals 50a, 50b. In some variants, the one or more line scan cameras 44 can be configured to record a plurality of images of the surface of the layer stack 24 while the surface is illuminated from the different directions, i.e. angle-selectively. In addition to the arrangement illustrated in FIG. 1, the device 10 could furthermore have a ring light having a plurality of light sources that are drivable individually and/or in segments and/or a plurality of light sources around the production platform 12 in order to enable an illumination of the surface of the layer stack 24 from different directions.

[0058] In some example embodiments, the device 10 can include a pattern generator 52, which can generate a bright-dark pattern 54 above the layer stack 24. In some example embodiments, the pattern generator can furthermore be configured to illuminate the surface of the layer stack 24 from different directions. The pattern generator 52 can be a display, for instance an LED, LCD or OLED display, with the aid of which the evaluation and control unit 34 can generate variable patterns 54. In some example embodiments, the evaluation and control unit 34 can generate a stripe pattern 54 with the aid of the pattern generator 52, which stripe pattern is displaced in a plurality of steps relative to the surface of the layer stack 24 in order to enable an inspection of the material layer 18 and/or workpiece layer 16 according to the principle of phase shifting deflectometry. This method is inherently known to those skilled in the art for the inspection of reflective surfaces, for instance from WO 2009/083248 A1, which is incorporated by reference here referring to the explanations concerning the principle of phase shifting deflectometry.

[0059] FIG. 2 shows one example embodiment of an inspection sensor 42, which can be arranged on the doctor blade 22 or alternatively on a separate connecting piece (not illustrated here). Identical reference signs designate the same elements as before.

[0060] The inspection sensor in accordance with FIG. 2 has a first line scan camera 44a and a second line scan camera 44b and also a line light source 46, which are held stationary with respect to one another on a suitable carrier 56. The first line scan camera 44a has an optical axis 58a oriented substantially orthogonal with respect to the surface 60 of the layer stack 24. The optical axis 58a of the first line scan camera 44a thus also runs orthogonally relative to the respective topmost material layer 18 and workpiece layer 16. In some example embodiments, the first line scan camera 44a can include optical elements 62a, such as lens and/or mirror elements, for instance.

[0061] The second line scan camera 44b has a second optical axis 58b, which here is arranged obliquely with respect to the surface 60 of the layer stack 24. In various embodiments, the optical axes 58a, 58b intersect along a line which is indicated at the point 64 in FIG. 2 and which runs parallel to the longitudinal extent of the line scan cameras 44a, 44b on the surface 60 of the layer stack 24. In some example embodiments, the line scan camera 44b also has optical elements 62b. In some example embodiments, it is preferred if the line of intersection 64 of the optical axes 58a, 58b substantially corresponds to the respective focal line of the line scan cameras 44a, 44b. In other example embodiments, the line of intersection 64 of the optical axes 58a, 58b can be at a defined distance from the focal line of the line scan cameras 44a, 44b, which is advantageous in particular if the surface 60 of the layer stack 24 is inspected by deflectometry.

[0062] The inspection sensor 42 in accordance with FIG. 2 furthermore has a line light source 46, which generates a light line along the surface 60. In some example embodiments, the line light source 46 can be a laser light source, which generates a laser line on the surface 60. In various embodiments, the light line extends over the line of intersection 64 of the optical axes 58a, 58b. Furthermore, in this example embodiment, it is preferred for the line light source 46 and the second line scan camera 44b to be arranged approximately axially symmetrically with respect to the optical axis 58a of the first line scan camera 44a. In various embodiments, a light beam that is emitted by the line light source 46 and is reflected at the surface 60 in the region of the line of intersection 64 impinges on the second line scan camera 58b according to the principle of angle of incidence=angle of reflection. Accordingly, the second line scan camera 58b can be used here in order to inspect the defined material layer 18 and/or workpiece layer 16 on the basis of triangulation relationships between the line light source 46 and the line scan camera 44b. The second line scan camera 58b in these example embodiments may capture light from the visible spectral range. In some example embodiments, the second line scan camera 58b can have a bandpass filter 66b, the pass frequency of which is in the visible spectral range. Additionally or alternatively, the line scan cameras 44a and/or 44b can record a plurality of images of the defined material layer 18 and/or workpiece layer 16 in order, together with the pattern 54 from the pattern generator 52, to realize an inspection on the basis of phase shifting deflectometry.

[0063] In various embodiments, the first line scan camera 44a can be an infrared camera or a linear array of pyrometers. Advantageously, the first line scan camera 44a here captures thermal radiation from the surface 60. Accordingly, the first line scan camera 44a can be used in order to monitor the shape and/or dimensions of the melt pool, for example. Additionally or alternatively, the first line scan camera 44a can be used in order to capture transient temperature signals from the surface 60. By way of example, an anomaly or a defect 68 in workpiece layers near the surface can be detected with the aid of a spatially resolved thermal image.

[0064] In some example embodiments, the line light source 46 can generate light in the visible spectral range and in the infrared range, in particular in the near infrared range, in order to enable firstly a thermal inspection with the aid of the first line scan camera 44a and secondly a visual/deflectometric inspection with the aid of the second line scan camera 44b. By way of example, the line light source can therefore emit light from the spectral range between 350 nm and 3000 nm.

[0065] In various embodiments, the device 10 has one or more multi-line scan cameras. A multi-line scan camera in this sense is a line scan camera having a plurality of camera elements in the movement direction 29. FIG. 3 shows by way of example one such multi-line scan camera 44, which is moved relative to the production platform 12 over the defined material layer 18 in the direction of the arrow 29. The reference numeral 70 indicates a measurement region, here having a length of 10 camera elements/pixels, for example, in the movement direction 29. In other words, the multi-line scan camera 44 in this example embodiment has ten camera elements in the movement direction 29 and a plurality of camera elements transversely with respect thereto. In various embodiments, the line scan cameras 44a, 44b used or a line scan camera 44 used extend(s) transversely with respect to the movement direction 29 in each case over the entire production platform 12. In the movement direction 29, the one or more line scan cameras here each capture a limited portion of the defined material layer 18.

[0066] In the example embodiment in accordance with FIG. 3, it is possible to illuminate a small region 74 in the movement direction 29 with the aid of a line light source of the type mentioned above, while a larger measurement region 70 is captured with the aid of the line scan camera 44. In these example embodiments, it is possible to capture transient signals in the measurement region 70 owing to a thermal excitation in the region 74. With the aid of the position signal from the encoder 48, it is possible to determine a respective instantaneous position of the line scan camera 44 relative to the material surface 18 and it is thus possible to synchronously record a plurality of spatially resolved images of the material surface during a movement of the inspection sensor. Accordingly, a respective scan image can be recorded by each line of camera elements 72, the different scan images differing by virtue of a respective different distance from the illumination in the region 74.

[0067] FIG. 4 shows a further example embodiment with an inspection sensor 42 arranged on a doctor blade 22. In this example embodiment, the doctor blade 22 has a reservoir 76 for receiving the particulate material 20 and also an outlet opening 78 for discharging the material 20. Furthermore, the inspection sensor has a first sensor part 42a and a second sensor part 42b. The sensor parts 42a, 42b can each include one or more line scan cameras and one or more line light sources (not illustrated separately here). In the example embodiment illustrated, the first sensor part 42a is arranged upstream of the material outlet 78 in the movement direction 29, while the second sensor part 42b is arranged downstream of the material outlet 78 in the movement direction 29. The second sensor part 42b can be used advantageously here in order to inspect a freshly produced powder layer. By contrast, the first sensor part 42a can be used advantageously in order to inspect a workpiece layer 16 produced last before a new powder layer is applied.

[0068] As illustrated in FIG. 5, in some example embodiments, the inspection sensor 42 can have a line scan camera 44 combined with a first line light source 46a and a second line light source 46b. In the example embodiment illustrated, the line scan camera 44 and the line light sources 46a, 46b each extend parallel to one another and, in various implementations, over the entire width of the production platform 12. In the movement direction 29, a line light source 46b leads the line scan camera 44, while a further line light source 46a trails behind the line scan camera 44. The line light sources 46a, 46b here have respective light cones 80a, 80b, which overlap in the region of the focal line 82 of the line scan camera 44. Accordingly, a region of the material layer 18 that is recorded by the line scan camera 44 can optionally be illuminated by the line light source 46a and/or by the line light source 46b. In particular, in these example embodiments, it is also possible for a region of the material layer 18 that is captured by the line scan camera 44 to be illuminated from opposite directions. A method for inspecting the defined material layer can advantageously be implemented in this way, as described below for one various embodiments with reference to FIGS. 7 and 8.

[0069] FIG. 6 shows an example embodiment of the inspection sensor 42 in accordance with FIG. 5 comprising a line scan camera 44 embodied as a multi-line scan camera having a plurality of camera elements 72, the camera elements 72 of the multi-line scan camera 44 each being sensitive to one of a plurality of light colors. By way of example, the multi-line scan camera 44 can have camera elements 72 which are sensitive either to the light color green or to the light color red or to the light color blue. Accordingly, in this example embodiment, the line scan camera 44 is a color line scan camera, wherein the individual camera elements can form a Bayer pattern. The first line light source 46a and the second line light source 46b (cf. FIG. 5) can each have light elements 84, each of which dominantly emits a light color. By way of example, the line light sources 46a, 46b can thus include light elements 84 which dominantly emit either red or green or blue light. With such an inspection sensor, the defined material layer 18 can be illuminated simultaneously from different directions, wherein the angle-selective illumination can be differentiated from one another on the basis of the light colors.

[0070] In some example embodiments, it is possible to use an inspection sensor in accordance with FIGS. 5 and 6 with an angle-selective illumination in an arrangement in accordance with FIG. 2 and/or FIG. 4. In further example embodiments, the line scan camera 44 and/or the line light sources 46a, 46b in accordance with FIGS. 5 and 6 can have camera elements 72 and/or light elements 84 which dominantly record and/or emit light in the infrared range. Accordingly, the variants illustrated here with reference to FIGS. 2 to 6 can be wholly or partly combined with one another.

[0071] The function of the novel device in accordance with an example embodiment is explained below with reference to FIGS. 7 and 8. In accordance with step 100, a data set 38 defining the workpiece 14 in a plurality of workpiece layers 16 arranged one on top of another is read into the evaluation and control unit 34. In accordance with step 102, a material layer 18 is produced on the layer stack 24. By way of example, for this purpose, the evaluation and control unit 34 can take particulate material 20 from the reservoir 28 with the aid of the doctor blade 22 and can distribute it on the layer stack 24. The distribution of the particulate material 20 is usually intended to be uniform and as homogeneous as possible.

[0072] In accordance with step 104, the surface of the new material layer 18 is inspected with the aid of the inspection sensor 42 in order to recognize possible inhomogeneities such as waves, accumulations, doctor blade structures or density variations in the particulate material. If the surface of the new material layer 18 satisfies all desired criteria, in accordance with step 106 the method branches to step 108, in which a new workpiece layer 16 is produced in the topmost material layer 18 with the aid of the structuring tool 30. If the surface of the new material layer 18 does not satisfy the desired criteria, in accordance with loop 110 the method branches back to step 102 in order to rework the surface of the new material layer 18 or produce it completely anew. In accordance with step 114, the manufactured workpiece layer 16 can be inspected. In accordance with step 112, steps 102-114 are repeated until the workpiece 14 is completed according to the data set 38.

[0073] FIG. 8 shows one advantageous example embodiment of method step 104 from FIG. 7. In accordance with step 116, the inspection sensor 42 is moved relative to the production platform 12 and the object surface of the material layer 18 is illuminated from a plurality of different directions in the process, for example with the aid of the line light sources 46a, 46b in accordance with FIG. 6. Furthermore, a plurality of images of the object surface illuminated in each case are recorded in scanning fashion here with the aid of the multi-line scan camera 44. In various embodiments, at least three images are recorded, although a higher number may be advantageous. Accordingly, in some example embodiments, the line scan camera 44 can have 3 or more lines having camera elements in the movement direction. Hereinafter, a first image is designated by 1 and a further k images are designated by J.sub.k, where the index k denotes the number of further images.

[0074] After all the images have been recorded, step 118 involves determining a maximum brightness value MH over all the pixel values of the recorded images. The maximum brightness value MH can be used later in the determination of an aggregate reflection image and/or else in the determination of a shadow-corrected image and/or a reflection-corrected image of the object surface in order to be able to consider the pixel values in the images as relative brightness values, for example between 0 and 1. In some example embodiments, step 118 can be omitted and the pixel values of the images can be used as absolute values.

[0075] A shadow-corrected image SC of the object surface is determined in accordance with steps 120 to 130, while an aggregate reflection image RI is determined in steps 132 to 138 and in addition a reflection-corrected image RC of the material layer 18 is also determined in step 140. Steps 120 to 130 and steps 132 to 140 are illustrated in parallel with one another in FIG. 8 and can be carried out in parallel with one another if the evaluation and control unit 34 enables a corresponding parallel processing. As an alternative thereto, steps 120 to 130 and steps 132 to 140 can be carried out successively and it is likewise possible to determine only an aggregate reflection image in accordance with steps 132-140 in order subsequently to determine individual properties of the material layer 18.

[0076] In accordance with step 120, a first indicator value US.sub.k and a second indicator value VS.sub.k are determined. Advantageously, the indicator value US.sub.k can be determined as follows:

US.sub.k=1(I−J.sub.k≤0).Math.1(J.sub.k−I≤MH.Math.0.1).Math.(J.sub.k−I)/0.1/MH+1(J.sub.k−I>0.1.Math.MH).

[0077] where the function 1 ( . . . ) yields the value 1 if the condition specified between the parentheses is met, and yields the value 0 if the condition between the parentheses is not met. The indicator value US.sub.k is determined for all corresponding pixel pairs of the images I, J.sub.k which are intended to be evaluated in the determination of the shadow-corrected image SC, and has the value 1 if a considered pixel in an image J.sub.k is brighter than the corresponding pixel in the image I by more than 10%. In this case, the percentage value relates to the maximum brightness value MH and it forms a threshold value that defines a limit of a brightness value range.

[0078] The threshold value with the magnitude of 10% that is assumed here is by way of example and can be chosen to be smaller or larger in other example embodiments. For relative brightnesses of a considered pixel in the image J.sub.k between 0 and the assumed threshold value, a continuous transition is realized by means of the above formula. Broadly speaking, the indicator value US.sub.k with the value 1 signals when a considered pixel in an image J.sub.k is brighter than the corresponding pixel in the first image I, where the threshold value of 10% ensures that there is in each case a “distinct” difference in brightness between the image pixels considered.

[0079] The second indicator value VS.sub.k is advantageously calculated as follows:

VS.sub.k=1(0.1≤BL−J.sub.k/MH)+1(0.1≥BL−J.sub.k/MH).Math.1(J.sub.k/MH≤BL).Math.(1+((BL−0.1)−J.sub.k/MH)/0.1),

[0080] where BL (denoting Bright Limit) defines a further threshold value. The indicator value VS.sub.k has the value 1 if the considered pixel in the image J.sub.k has a relative brightness value of between at least 0 and a maximum value, which here is for example 10% of MH below the threshold value BL MH. Here, too, a continuous link is advantageously realized in the transition to the upper threshold value BL. Broadly speaking, the indicator value VS.sub.k assumes the value 1 if the pixel value of a considered pixel in an image J.sub.k is between 0 and the upper threshold value defined by BL.

[0081] In accordance with step 122, a combined indicator value WS.sub.k is determined in accordance with the following formula

WS.sub.K=(US.sub.k.Math.VS.sub.k).sup.1/2

[0082] from the indicator values mentioned above. The combined indicator value WS.sub.k prevents a considered pixel in an image J.sub.k from being included in the further calculation if it is too bright or too dark. The exponent 1/2 here represents a square root operation, which has been found to be advantageous for the inspection of a metal powder. In principle, however, the product can also remain without a further operation or be exponentiated with other exponents p<1. Furthermore, there may be applications in which exponents p>1 are better. Depending on whether the exponent p>1 is involved, a profile with values close to zero is smoothed, or, in the case of exponents p<1, a profile with values close to 1 is smoothed. An exponent p>0 is thus applicatively possible. An exponent 0<p≤1 is particularly advantageous.

[0083] In accordance with step 124, a difference RS.sub.k is then formed for all considered pixel pairs in the images I and J.sub.k, this difference being weighted with the combined indicator value WS.sub.k in accordance with the following formula

RS.sub.k=(I−J.sub.k).Math.WS.sub.k

[0084] The difference values RS.sub.k each form a correction vector for a pixel in the first image, which correction vector is weighted with the combined indicator value WS.sub.k.

[0085] In various embodiments, the correction vectors RS.sub.k are averaged in order to determine a shadow-corrected image SC. For this reason, in step 126, a factor Z is determined in accordance with the following formula:

Z=1/sum(WS.sub.k,k).

[0086] The correction vectors are advantageously averaged here with the aid of the sum of the combined indicator values WS.sub.k. In accordance with step 128, a damping factor is advantageously determined in the form of the maximum combined indicator value over all combined indicator values WS.sub.k, i.e.

M=max(WS.sub.k,k)

[0087] In accordance with step 130, a shadow-corrected image is then determined in accordance with the following formula:

SC=I−Gaussfilt(sum(RSk,k).Math.M,1).

[0088] In this case, the function Gaussfilt ( . . . , 1) denotes a Gaussian filter with a standard deviation, here assumed to be 1, for example. In principle, the value for the standard deviation could be chosen differently.

[0089] Accordingly, in step 130, a shadow-corrected image is determined by determining a difference between the pixel values in the first image I and corresponding pixel values in a weighted and filtered correction image. The correction image results here from the Gaussian-filtered sum of all correction vectors, where the correction vectors are in turn weighted with the combined indicator value WS.sub.k. Some other local smoothing filter could also be used as an alternative to Gaussian filtering. The threshold value BL achieves the effect that the reflections in the images I, J.sub.k do not add up in an undesired manner in the determination of the shadow-corrected image SC. The filtering in the final step 130 avoids or reduces artificial artefacts. Abrupt changes in the summed correction are largely avoided by means of the multiplication of the summed correction vectors by the maximum of the combined indicator value.

[0090] The determination of an aggregate reflection image and—in some instances—a reflection-corrected image is carried out in a similar manner. Step 132 involves firstly calculating an indicator value UR.sub.k in accordance with the following formula:

URk=1(Jk−I≤0).Math.1(I−Jk≤MH.Math.0.1).Math.(I−Jk)/0.1/MH+1(I−Jk>0.1.Math.MH).

[0091] Accordingly, a considered pixel in the images J.sub.k influences the correction of the first image I if its brightness value is darker than the brightness value of the corresponding pixel in the first image I. Furthermore, step 134 involves calculating an indicator value VRk in accordance with the following formula:

VRk=1(Jk/MH>DL+0.1)+1(Jk/MH>DL).Math.1(Jk/MH≤DL+0.1).Math.(J.sub.k/MH-DL)/0.1.

[0092] The considered pixel in the images J.sub.k is intended to have a minimum brightness, which is set here by the threshold value DL (denoting Dark Limit).

[0093] In other words, a considered pixel in the images J.sub.k is intended to be in a second brightness value range, which, in accordance with step 134, is in turn defined with the aid of a combined indicator value WR.sub.k. By way of example, the combined indicator value WR.sub.k is calculated in accordance with the following formula:

WRk=(URk.Math.VRk).sup.1/2

[0094] In accordance with step 136, the corresponding pixel values in the first image I and in the further images J.sub.k are subtracted from one another for all k and weighted with the combined indicator value WR.sub.k. In other words, step 136 involves calculating correction vectors RR.sub.k in accordance with the following formula:

RR.sub.k=(I−J.sub.k).Math.WR.sub.k.

[0095] Step 138 then involves calculating an aggregate reflection image by determining the maximum of the correction vectors RR.sub.k over all k images:

RI=max(RRk,k).

[0096] The aggregate reflection image is thus a substitute image calculated from the images I, J.sub.k and substantially showing the reflections at the surface of the material layer 18. This reflection image shows the spatial distribution of the light reflections at the material surface and is thus representative of the homogeneity of the distribution of the material particles.

[0097] In step 140, here advantageously a reflection-corrected image is also calculated according to the following formula:

RC=I−RI.

[0098] In other words, the reflection-corrected image is calculated by subtracting the pixel values of the aggregate reflection image from the pixel values of the first image I. Since the aggregate reflection image represents the reflections at the surface, the light reflections in the first image are largely eliminated by the subtraction. During the reflection correction, here—in contrast to the shadow correction—the maximum effect is advantageously used since reflections are typically locally limited and, in some instances, also greatly overmodulated. Alternatively, however, a Gaussian filter can be employed here in a manner corresponding to the case of shadow correction.

[0099] Step 142 involves determining individual properties of the material surface 18 such as height variations (for instance on account of doctor blade structures, waves, material accumulations or other unevennesses), individual particle sizes of the powder particles and/or the homogeneity of the distribution of the powder particles. By way of example, here it is possible to check whether the spatial distribution of the light reflections in the aggregate reflection image satisfies a defined uniformity criterion and/or whether the material surface 18 in the reflection-corrected and/or shadow-corrected image satisfies a defined criterion with regard to smoothness or possible height variations. If this is the case, a workpiece layer 16 is produced in accordance with step 108 (FIG. 7).

[0100] In principle, the inspection, in particular the determination of an aggregate reflection image and/or of a height map, can be carried out after the production of the workpiece layer, which is indicated at the reference numeral 114 in FIG. 7. Depending on the spatial distribution of light reflections at the workpiece layer produced, the workpiece can be reworked in order to correct possible defects, or the partly finished workpiece is rejected if it has defects which do not give reason to expect a successful correction, for economic and/or technical reasons.

[0101] As already indicated above, as an alternative or in addition to the method in accordance with FIG. 8, it is possible to carry out an inspection of the workpiece layer 16 with the aid of phase shifting deflectometry using the pattern generator 52, on the basis of a simple deflectometric examination of the workpiece surface using an inspection sensor in accordance with FIG. 2 and/or on the basis of a thermographic analysis of the workpiece surface. The phrase “at least one of A, B, and C” should be construed to mean a logical (A OR B OR C), using a non-exclusive logical OR, and should not be construed to mean “at least one of A, at least one of B, and at least one of C.”