Method and apparatus for calibrating optical elements

1. A calibration method for calibrating one or more optical elements of an additive layer manufacturing apparatus useable for producing a three-dimensional workpiece, the method comprising:

projecting, using the one or more optical elements, an optical pattern onto a material in order to prepare, from said material, solidified material layers using an additive layer manufacturing technique to form a test sample;

determining a geometry of the test sample;

comparing the determined geometry with a nominal geometry to generate calibration data; and

calibrating the one or more optical elements using said calibration data,

wherein one or more of the material layers of the test sample are prepared only when a first temperature variation rate of a first temperature of a said optical element and/or a second temperature variation rate of a second temperature of an enclosure in which the one or more material layers are prepared and/or a third temperature variation rate of a third temperature of a substrate on which the test sample is formed is below a threshold rate.

2. A calibration method as claimed in claim 1 , wherein the geometry and the nominal geometry comprise coordinates and nominal coordinates, respectively, of a location of the test sample.

3. A calibration method as claimed in claim 1 , wherein said determining comprises determining geometrical data relating to a said material layer of the test sample prepared last using the additive layer manufacturing technique.

4. A calibration method as claimed in claim 1 , wherein said preparing using the additive layer manufacturing technique comprises supplying different layers of said material on a same height level for preparing the test sample using the additive layer manufacturing technique.

5. A calibration method as claimed in claim 4 , wherein said supplying of the different layers of said material on the same height level comprises lowering the test sample prior to supplying consecutive ones of the different layers.

6. A calibration method as claimed in claim 1 , wherein said forming of the test sample is performed over a period longer than a threshold period.

7. A calibration method as claimed in claim 1 , wherein the nominal geometry is based on data used for defining the optical pattern.

8. A calibration method as claimed in claim 1 , wherein the substrate is lowerable during the forming of the test sample.

9. A calibration method as claimed in claim 1 , wherein a substrate material of the substrate on which the test sample is formed is identical with the material used to form the test sample.

10. A calibration method as claimed in claim 1 , wherein the third temperature of the substrate is maintained at a target temperature and/or within a target temperature range during one or both of (i) the forming of the test sample and (ii) the determining of the geometry of the test sample.

11. A calibration method as claimed in claim 1 , wherein a build height of the test sample is above a threshold height which is dependent on an amount of heat transferable, via the test sample, from an area of the material onto which the optical pattern is projected to an opposite side of the material.

12. A calibration method as claimed in claim 1 , further comprising taking an optical image of the test sample, and wherein said determining of the geometry of the test sample comprises analyzing the optical image.

13. A calibration method as claimed in claim 12 , further comprising:

placing a real and/or virtual grid over the optical image and/or placing one or more reference marks on a floor of an enclosure, in which the test sample is formed, and

identifying, in the optical image, a location of the test sample relative to the grid and/or the one or more reference marks to determine the geometry of the test sample; and/or

wherein the optical image is taken upon the forming of the test sample having been completed.

14. A calibration method as claimed in claim 1 , wherein the determining of the geometry of the test sample is performed using a coordinate-measuring machine.

15. An apparatus for producing a three-dimensional workpiece, the apparatus comprising:

a substrate adapted to receive material useable for producing the three-dimensional workpiece;

one or more optical elements adapted to project an optical pattern onto the material in order to prepare, from said material, solidified material layers using an additive layer manufacturing technique to form a test sample; and

a calibration unit adapted to:

determine a geometry of the test sample;

compare the determined geometry with a nominal geometry to generate calibration data; and

calibrate the one or more optical elements using said calibration data,

wherein the apparatus is configured to prepare one or more of the material layers of the test sample only when a first temperature variation rate of a first temperature of a said optical element and/or a second temperature variation rate of a second temperature of an enclosure in which the one or more material layers are prepared and/or a third temperature variation rate of a third temperature of the substrate is below a threshold rate.

16. An apparatus as claimed in claim 15 , further comprising a heating unit comprising a heating frame, wherein the heating unit is thermally coupled to the substrate to heat the substrate to and/or maintain the substrate at a target temperature and/or a target temperature range.

17. An apparatus as claimed in claim 15 , further comprising:

one or more temperature sensors adapted to measure the temperature variation rate of the temperature of one or more of (i) the one or more optical elements, (ii) the substrate, and (iii) the enclosure of the apparatus in which the substrate is arranged;

a controller for controlling the one or more optical elements, wherein the one or more temperature sensors are coupled to the controller; and

wherein the controller is adapted to control the one or more optical elements to project the optical pattern onto the material in order to prepare the one or more material layers of the test sample only when the first temperature variation rate and/or the second temperature variation rate and/or the third temperature variation rate is below the threshold rate.

18. An apparatus as claimed in claim 15 , wherein the nominal geometry is based on data used for defining the optical pattern.

19. A calibration method for calibrating one or more optical elements of an additive layer manufacturing apparatus useable for producing a three-dimensional workpiece, the method comprising:

projecting, using the one or more optical elements, an optical pattern onto a material in order to prepare, from said material, solidified material layers using an additive layer manufacturing technique to form a test sample;

determining a geometry of the test sample;

comparing the determined geometry with a nominal geometry to generate calibration data; and

calibrating the one or more optical elements using said calibration data,

wherein said preparing using the additive layer manufacturing technique comprises supplying different layers of said material on a same height level for preparing the test sample using the additive layer manufacturing technique.

20. A calibration method for calibrating one or more optical elements of an additive layer manufacturing apparatus useable for producing a three-dimensional workpiece, the method comprising:

projecting, using the one or more optical elements, an optical pattern onto a material in order to prepare, from said material, solidified material layers using an additive layer manufacturing technique to form a test sample;

determining a geometry of the test sample;

comparing the determined geometry with a nominal geometry to generate calibration data; and

calibrating the one or more optical elements using said calibration data,

wherein a build height of the test sample is above a threshold height which is dependent on an amount of heat transferable, via the test sample, from an area of the material onto which the optical pattern is projected to an opposite side of the material.

21. A calibration method for calibrating one or more optical elements of an additive layer manufacturing apparatus useable for producing a three-dimensional workpiece, the method comprising:

projecting, using the one or more optical elements, an optical pattern onto a material in order to prepare, from said material, solidified material layers using an additive layer manufacturing technique to form a test sample;

determining a geometry of the test sample;

comparing the determined geometry with a nominal geometry to generate calibration data;

calibrating the one or more optical elements using said calibration data;

taking an optical image of the test sample, and wherein said determining of the geometry of the test sample comprises analyzing the optical image;

placing a real and/or virtual grid over the optical image and/or placing one or more reference marks on a floor of an enclosure, in which the test sample is formed, and

identifying, in the optical image, a location of the test sample relative to the grid and/or the one or more reference marks to determine the geometry of the test sample; and/or

wherein the optical image is taken upon the forming of the test sample having been completed.