Method for detecting a defect on a surface by multidirectional lighting and associated device

1. A method for detecting a defect on a surface, the method comprising:

acquiring a plurality of images of the surface using an optical device having an optical axis, each image being acquired with a lighting of the surface along a lighting direction given for each point of the surface and with the optical axis of the optical device along a given optical direction, the images being acquired with different lighting directions or different combinations of lighting directions and/or with different optical directions;

for each point of the surface, calculating, from the acquired images, a plurality of parameters, the parameters including coefficients of an equation characterizing response of the point of the surface as a function of the lighting direction and an observation direction;

wherein, for each point of the surface, the calculation of the parameters includes: defining a quantity evolving as a function of the acquired images; choosing a model of the surface depending on the lighting direction(s) and/or the observation direction for the evolution of the quantity, the model including the coefficients, and calculating coefficients by regression of the evolution of the quantity; and

deducing from the calculated parameters whether the surface has a defect at the point.

2. The detection method according to claim 1 , wherein the optical direction is the same for the plurality of images.

3. The detection method according to claim 2 , wherein the optical direction is substantially perpendicular to the surface.

4. The detection method according to claim 1 , wherein the lighting is done by a lighting device, the lighting device including a first number of light sources, the first number being greater than six, each light source having a different lighting direction,

for each acquired image, a single light source or a defined combination of sources being illuminated, the single light source or combination being different for each acquired image.

5. The detection method according to claim 4 , wherein the light sources have an identical light intensity.

6. The detection method according to claim 4 , wherein the light sources are arranged in a half-sphere surrounding the surface.

7. The detection method according to claim 4 , wherein the lighting device includes one or several light sources that are configured for moving in a first number of positions relative to the surface.

8. The detection method according to claim 1 , wherein the quantity corresponds to the gray intensity of the point of the surface, the model depending on the lighting direction.

9. The detection method according to claim 1 , wherein the defect is detected at the point when at least one parameter at the point is not comprised in a defined interval.

10. The detection method according to claim 1 , wherein the defect is detected at the point when at least one parameter at the point is not comprised in an interval centered around the mean value of the parameter on the set of points of the surface.

11. The detection method according to claim 1 , wherein the defect is detected at the point when at least one parameter at the point differs from a value chosen as detection threshold relative to a background noise of the parameter at an adjacent point, the surface not having a defect at the adjacent point.

12. A device for detecting a defect on a surface, the detection device including:

an optical device having an optical axis, the optical device being configured to acquire an image of the surface along a given optical direction;

a lighting device having several different lighting direction; and

an electronic calculating device, the electronic calculating device being configured to:

acquire a plurality of images of the surface via the optical device, each image being acquired along the given optical direction with different lighting directions or different combinations of lighting directions for each image,

for each point of the surface, calculate, from acquired images, a plurality of parameters, the parameters including coefficients of an equation characterizing the response of the point of the surface as a function of the lighting direction and an observation direction;

wherein, for each point of the surface, the calculation of the parameters includes: defining a quantity evolving as a function of the acquired images; choosing a model of the surface depending on the lighting direction(s) and/or the observation direction for the evolution of the quantity, the model including the coefficients, and calculating coefficients by regression of the evolution of the quantity; and

deduce from the calculated parameters whether the surface has a defect at the point.