Method for monitoring the energy density of a laser beam by image analysis and corresponding device

1. A method for controlling the energy density of a laser beam from at least two parameters of the laser beam, the method comprising:

repeatedly applying the laser beam on a reference support and measuring, at each application of the laser beam on the reference support, the light intensity obtained on the reference support,

identifying a variation in the light intensity on the reference support between at least two measurements, and

when the variation in the light intensity is greater than a threshold, determining at least one unstable parameter of the energy density of the laser beam.

2. The control method according to claim 1 , wherein the energy density of the laser beam is controlled from three parameters comprising the power, the scanning velocity and the size of the laser beam.

3. The control method according to claim 1 , wherein the repeatedly applying the laser beam and measuring the light intensity on the reference support comprises:

(i) applying the laser beam on the reference support and acquiring an image of the laser beam on said reference support, in order to obtain a reference image,

(ii) determining the light intensity of the laser beam in the reference image, and

(iii) repeatedly applying the laser beam on the reference support and acquiring, at each application, an image of the laser beam on said reference support, in order to obtain a control image, and determine the light intensity on the reference support of the laser beam in the control image, and

the identifying the variation in the light intensity comprises:

(iv) comparing the light intensity of an obtained control image with the light intensity of the reference image, and

(v) inferring therefrom the variation in the energy density of the laser beam.

4. The control method according to claim 3 , wherein the light intensity is determined by measuring the greyscale of the reference image and of the control image.

5. The control method according to claim 4 , wherein the greyscale of the reference image and of the control image are measured in several points, so that the light intensity of the reference image and of the control image is determined by averaging the greyscale in each point of the reference image and of the control image, respectively.

6. The control method according to claim 3 , further comprising, before applying the laser beam on the reference surface in order to acquire the reference image, determining an initial value of each of the parameters, and further comprising, when the variation in the light intensity is greater than the threshold,

determining values of the parameters of the laser beam and comparing the determined values to the initial values of said parameters in order to identify the at least one unstable parameter, and

modifying a laser generating the laser beam in order to re-stabilize said at least one unstable parameter.

7. The control method according to claim 6 , wherein the steps (i) to (iii) are repeated after the laser is modified, in order to define a new reference image.

8. The control method according to claim 7 , wherein the determining an initial value of each of the parameters is repeated after the laser is modified.

9. A device for controlling the energy density of a laser beam from at least two parameters of the laser beam, said parameters comprising a power, a scanning velocity and a size of the laser beam, the device controlling the energy density of the laser beam comprising:

an image acquisition system, configured to acquire images of the laser beam each time the laser beam is applied on a reference support, and

a first computer, configured to measure, each time the laser beam is applied on the reference support, the light intensity of the images acquired by the image acquisition system and to identify a variation in the light intensity on the reference support between at least two measurements, and

a second computer configured to determine, when the variation in the light intensity is greater than a threshold, at least one unstable parameter of the parameters of the laser beam.

10. The control device according to claim 9 , wherein the reference support comprises a homogenous coating.

11. The control device according to claim 9 , wherein the reference support comprises a plate in aluminum alloy.

12. The control device according to claim 11 , wherein the plate in aluminum alloy is anodized and comprises an anodization layer.

13. The control device according to claim 12 , wherein the anodization layer is of a black color.

14. The control device according to claim 12 , wherein the anodization layer has a thickness comprised between 0.5% and 3% of a thickness of the plate in aluminum alloy.

15. The control device according to claim 14 , wherein the thickness of the plate in aluminum alloy is of the order of 1 mm and the thickness of the anodization layer is of the order of 0.01 mm.