Method of generating a map for use in a laser engraving process and laser engraving method

1. A method of generating a map for use in a laser engraving process using a laser engraving apparatus, the method comprising:

providing a plurality of calibration images having mutually different, uniform grayscale values between a minimum grayscale value and a maximum grayscale value;

engraving the plurality of calibration images on a predefined substrate surface of a substrate by varying only a single laser parameter of the laser engraving apparatus;

measuring a colour value of each of the engraved calibration images and of the predefined substrate surface in a colour space;

calculating a plurality of colour space distances between each of the colour values of the engraved calibration images and the colour value of the substrate surface;

for at least a subset of the colour space distances, associating each colour space distance with one of a plurality of predefined laser parameter values between a minimum laser parameter value and a maximum laser parameter value, in accordance with at least one of the grayscale values or the colour space distances of the corresponding calibration images;

determining a relationship between colour space distance and laser parameter value based on the associations; and

generating the map by determining a colour space distance for a grayscale value of an input image, determining a corresponding laser parameter value based on the determined relationship, determining a grayscale value of an output image based on a predefined relationship between the determined laser parameter value and the grayscale value of the output image, and mapping the grayscale value of the input image to the grayscale value of the output image.

2. The method of claim 1 , further comprising:

prior to engraving the plurality of calibration images, engraving at least one of the calibration image having the minimum grayscale value and the calibration image having the maximum grayscale value; and

in case the substrate surface is physically damaged by engraving the calibration image having the minimum grayscale value, or in case the calibration image having the maximum grayscale value can be distinguished from the substrate surface, adjusting at least one laser parameter of the laser engraving apparatus until the substrate surface is no longer physically damaged by engraving the calibration image having the minimum grayscale value, or the calibration image having the maximum grayscale value cannot be distinguished from the substrate surface.

3. The method of claim 1 , further comprising engraving two or more of the calibration images on the predefined substrate surface of a single substrate.

4. The method of claim 3 , wherein the two or more calibration images are included in a single data set to be engraved.

5. The method of claim 1 , wherein the single laser parameter is one of a laser power, a laser scan speed, a laser scan line distance, and a laser repetition rate.

6. The method of claim 5 , wherein the colour space is a CIELAB colour space, and the colour values are measured using a CIELAB spectrometer.

7. The method of claim 6 , wherein the colour space distance (ΔE) between the colour value of each engraved calibration image and the colour value of the substrate surface is calculated using the formula:

Δ

⁢

E

=

sqrt

(

(

L

*

-

L

ref

*

)

⁢

2

+

(

a

*

-

a

ref

*

)

⁢

2

+

(

b

*

-

b

ref

*

)

⁢

2

)

,

wherein L*, a*, and b* are values of the engraved calibration image in the CIELAB colour space, and L ref *, a ref *, and b ref * are values of the substrate surface in the CIELAB colour space.

8. The method of claim 1 , further comprising selecting a subset of the engraved calibration images, to which the at least a subset of colour space distances corresponds.

9. The method of claim 1 , wherein the determined relationship is determined by fitting a theoretical curve to a plurality of data points, each respectively indicating a predefined laser parameter value and its associated colour space distance.

10. The method of claim 9 , wherein the fitting is performed by a user by adjusting parameters of the theoretical curve.

11. The method of claim 10 , wherein the parameters of the theoretical curve include at least one of a curve slope, a slope position, a horizontal offset, or a vertical offset.

12. The method of claim 9 , further comprising determining a laser parameter threshold lower than the maximum laser parameter value corresponding to a predetermined colour space distance threshold, and using only data points below the laser parameter threshold in the fitting.

13. The method of claim 1 , wherein the predefined substrate surface is a predefined white background portion of the substrate, and the engraved calibration images are black and white grayscale images.

14. The method of claim 1 , wherein the predefined substrate surface is a predefined coloured background portion of the substrate, and the engraved calibration images are colour images.

15. A method of engraving an image on a substrate using a laser engraving apparatus, the method comprising:

receiving an input image;

if the input image is not a grayscale image, converting the input image into a grayscale image;

mapping a grayscale value of each pixel of the grayscale image onto a new grayscale value using a map generated for the laser engraving apparatus and the substrate using the method of claim 1 to generate a new grayscale image; and

engraving the new grayscale image on the predefined substrate surface of the substrate by varying the single laser parameter varied during the generation of the map.

16. The method of claim 15 , further comprising:

selecting the map from a plurality of maps generated for at least one of different substrates or different substrate surfaces.