The present invention relates to laser treatments of the surfaces of stainless steel sheets or other materials, intended to give these surfaces an iridescent appearance.

The iridescent treatment, also called "LIPPS" or "ripples" consists of irradiating the surface of a material with pulsed laser radiation of short pulse duration (less than one nanosecond). The diameter of each pulse at its place of impact on the material to be treated is typically of the order of 10 to a few hundred μm. If the energy of the incident beam is sufficiently high, this irradiation induces the modification of the structure and/or the reorganization of the surface of the material which will adopt a periodic structure. However, if the energy of the beam is too high, a phenomenon of ablation by vaporization/sublimation/shock wave can take place, preferentially or jointly with the formation of the periodic surface structure.

Such treatment is practiced, in particular, but not exclusively, on stainless steels of all types. The purpose of this treatment can be purely aesthetic, but it also allows to modify the wettability of the surface, and also its resistance to friction and to reduce bacterial adhesion. The treatment can be done directly on the surface of the object on which the stainless steel passivation layer is located without the need for prior activation/depassivation.

Other materials on which this treatment is practiced are, in particular, various metals, polymers such as PVC, ceramics, glass.

In the rest of the text, the case of stainless steels will be preferred, it being understood that the invention is applicable to all metallic or non-metallic materials which are currently, or would be known in the future, to be able to present an iridescent appearance continued to a laser treatment carried out as indicated, possibly by adapting the precise operating parameters of the installation (power and frequency of the lasers, etc.) which are known to play a role in obtaining the resulting iridescent appearance formation of a periodic surface structure.

Although the exact mechanism of formation of this periodic surface structure has not yet been determined, tests and characterizations carried out by different laboratories show that, depending on the number of laser passes and/or

pulse energy and/or scanning parameters, the structure of the surface may exhibit one of the following four structures, depending on the total irradiation energy per unit area, these structures being ranked in order of energy crescent and their denomination being usual for the person skilled in the art, even non-English speaking:

1 ) Structure known as “HSFL” (High Spatial Frequency LIPPS):

This structure is composed of small wavelets which, in the case of stainless steels, are oriented in the direction of the polarization of the incident laser beam. The spatial frequency of these wavelets is lower than the wavelength of the laser used for the treatment.

2) Structure known as “LSFL” (Low Spatial Frequency LIPPS):

This structure is composed of larger wavelets than the previous ones oriented, in the case of stainless steels, in the direction perpendicular to the polarization of the incident beam. The spatial frequency of these wavelets is slightly lower, or higher, or equal to the wavelength of the laser. For the treatment of a stainless steel surface with a laser of wavelength 1064 nm, the periodicity of the wavelets is of the order of 1 μm. It is still possible to see the HSFL structure in the depressions of the LSFL structure.

It will be noted that for certain materials, the respective orientations of the HSFL and LSFL structures can be reversed with respect to what they are for the stainless steels.

3) Structure called “Grooves” or “Bumps”:

This structure is made up of bumps of micrometric dimensions covering the entire treated surface. These bumps are organized according to a structure resembling a “snakeskin” appearance.

4) Structure in peaks or “spikes”:

This structure is composed of peaks whose height ranges from a few micrometers to a few tens of micrometers. The distance between the peaks depends on the processing parameters.

More details on these structures and the mechanism of their appearance can be found in the article "Evolution of nano-ripples on stainless Steel irradiated by picosecond laser pulses", Journal of Laser Applications 26, February 2014, by B. Liu et al. . It is notably said there that, for an equal number of pulses, an increase in the fluence of the irradiation leads to obtaining HSFLs rather than LSFLs (as we have just said), whereas for an equal fluence, a number higher pulses leads to the creation of LSFL rather than HSFL, until the number of pulses becomes too high for ripples to be observed. The exact configuration of the surface after irradiation therefore results from a mechanism involving both the number of pulses received and the energy delivered by each of them, for a given material.

In general, in the first two cases, this periodic organization of the surface allows an induced phenomenon, well known to practitioners of laser surface treatments, which is the diffraction of light by the creation of an optical network when the sample treated is placed under a light source. It is then possible to observe, depending on the orientations and positions of the user and the light, the colors of the rainbow on the sample. This is called an “iridescent appearance”.

This appearance no longer exists when the surface of the sample has a pronounced appearance according to the third and fourth cases mentioned above, because, in these two cases, the energy supplied by the laser source to the surface of the sample has reaches too high a level, at least locally, resulting in deformations of the surface which no longer allow the iridescent appearance to be obtained, because the structuring of the surface has lost its periodic character.

This iridescence should not be confused with the colorations of the surface of stainless steels which are obtained, voluntarily or involuntarily, by plasma treatments or surface oxidations due to a passage in an oven or by the passage of a blowtorch. The iridescent appearance referred to in the present invention does not result from a coloring of the material strictly speaking, but from the appearance of colors on the surface, under certain observation conditions. The absence of periodicity of the surface structure in the actual coloring processes is an essential difference between the iridescence of the surfaces to which the present invention relates and the coloring of stainless steels by plasma, baking or passing a blowtorch.

It should, however, be noted that the observation or not of such an iridescence is usually very directional, that is to say that the observation of this iridescence, and the intensity of the iridescence observed, are strongly dependent the angle from which the surface of the material is observed.

A problem faced by surface iridescence practitioners is the following.

It is currently possible to produce homogeneous samples in the laboratory with an iridescent treatment using either only a system coupling a laser and a scanner, realizing both a fast axis of scrolling of the laser beam (via a polygonal wheel or a galvanometer mirror) and a slow scrolling axis of the laser beam (via a galvo mirror), ie a laser and scanner system, coupled with a robotic arm carrying out the movement of the scanner along the slow axis with respect to the object to be treated.

Moving the scanner along the slow axis can be replaced by moving the object to be processed, such as sheet metal, along the slow axis, facing a laser which remains fixed along the slow axis and moves along the fast axis. Provision can also be made for the laser to remain fixed along the two axes (slow and fast), and for it to be the object to be treated which is moved along the two axes. The main thing is to have a relative movement between the object to be treated and the laser, successively along the two axes.

The formation mechanism of the structures that have been described depends on the total energy transferred to the surface of the material and on the spatial and temporal distribution of this energy. Thus, the “intensity” of the iridescence obtained thanks to the LSFLs will increase between each new passage of the laser on the passages already treated, until reaching a maximum, then it will decrease when the LSFLs will gradually transform into “Bumps ” as a result of the additional energy input.

This implies that there is an optimum of energy to be transferred to the surface of the material, an optimum for which the effect of iridescence is the most intense, and that this optimum must be determined and carried out over the whole of the surface concerned.

However, the treated objects are generally small and/or produced with low productivity.

The size limitation of the objects is mainly due to the limitation of the dimensions of the optical fields of the assemblies formed by the laser, the scanner and the focusing system, the latter possibly being, for example, a lens, or a converging mirror. Indeed, obtaining a homogeneous treatment requires perfect control of the treatment at all points of the surface. However, whatever the focusing systems used, they have an optical field on which they have a stable effect in an optimal zone. But as soon as one leaves this optimal zone, the system induces distortions and/or attenuations of the power of the laser beam. They result in a non-homogeneous treatment between the optimal zone of the optical field and the zones which are located beyond this optimal zone.

Thus, to treat large surfaces of stainless steel sheets, wide-field focusing systems would be required, which would be cumbersome and could only be produced on demand. In addition, it should be possible to use them in conjunction with high-power ultra-short pulse duration lasers, which are not yet widely available on the market.

To remedy this double drawback, the known solutions are to use conventional focusing systems and lasers currently available on the market, and:

- Either to place side by side several devices including these focusing systems and lasers in the case of in-line processing of a running strip;

- either to carry out the treatment in several times (according to a succession of strips distributed over the surface for a discontinuous system);

- or to combine these two solutions.

However, this solution requires particularly careful management of the junction zones between the optical fields of two successive devices, which, if they are poorly made, can cause a phenomenon called "stitching" by those skilled in the art, and that the we will describe.

This mechanism prevents having to resort to a very significant overlapping of the fields to join two consecutive laser treatment fields.

Indeed, if there is a very significant overlapping of the fields, which would be of the order of magnitude of the resolution of the human eye, this implies that the overlapping zone receives twice the quantity of energy transferred on the rest of the surface. This doubling of the energy injected during the treatment induces a local change in the structure, and therefore in the surface appearance, compared to the zones which have received only the nominal quantity of energy from the treatment, and this change is visible to the naked eye. It is this phenomenon that is commonly called "stitching", in that it can make the junction zone of the two fields visible.

Conversely, a separation of the laser treatment fields, which would certainly make it possible to avoid this phenomenon of local doubling of the treatment and the "stitching" which would result therefrom, would involve the formation, between the two fields, of an untreated zone, or which would, in any case, be treated less than normal. This area would also be visible to the naked eye.

It would therefore be necessary to achieve an almost perfect junction between the consecutive fields of laser treatment.

On the other hand, carrying out this type of high-productivity processing involves working at high scanner scanning speeds (which can go at least up to km/s). The scanning systems used for this type of processing are, most typically, scanners having at least one polygon wheel. At high laser frequencies and high scan speeds, these systems generally have synchronization problems between the laser electronics and the scanner electronics. These synchronization deviations induce a shift in the position of the first

draws from the line in relation to its target position, and therefore from the entire line. Although this difference is predictable and calculable (because it results from the difference in the management frequencies of the two devices), it is suffered in most current systems, and can represent a difference of a few tens of micrometers between the beginnings of the processing lines. (lines which are due to the movement of the polygon wheel). This deviation is a function of the rotational speed of the polygon and the natural frequency of the laser, and experience shows that an overlapping of the fields with such a deviation is already sufficient for the area where the treatment has been doubled to be able to influence the iridescent appearance of sheet metal.

Some systems under development have an internal means of partially correcting this shift, by the action of an additional deflecting mirror, called “galvo”, operating like a galvanometer, located upstream of the polygon. For example, the firm RAYLASE presented the concept of such a system at the SLT 2018 congress in Stuttgart on June 5 and 6, 2018: "New Generation of High-Speed ​​Polygon-Driven 2D Deflection Units and Controller for High-Power and High- Rep. Rate Applications” (presentation by E. Wagner, M. Weber and L. Bellini).

The improvement is, however, not by itself of sufficient quality for the undesirable effects of a shift in the fields to disappear with certainty. Indeed, the initial and final parts of each line run the risk of not being processed with the same energy input as the rest of the line. It is also necessary that the corresponding lines of two neighboring fields be strictly aligned.

To solve this local processing deficit, one can consider increasing the energy input on the rest of the line, but there is then a risk of exceeding the maximum energy input adapted to the creation of the LSFLs, and therefore of reducing or even remove iridescence. For all the other systems, this lack of synchronization implies a need for a “virtual” overlap of the order of at least twice the dispersion of the positions of the beginnings of lines between the different optical fields. Thus, this overlap results in a heterogeneous band where there are no untreated areas between the fields, but where there may be an overlap of twice this dispersion in places.

If the edges of each field are defined as "straight" and if, therefore, the length of each line of each field is constantly the same, as is a priori self-evident for those skilled in the art, the overlapping zone arises then as a thin rectilinear strip running through the part in the direction of relative movement of the part and the laser scanner devices, of width substantially equal to the width of the processing lines, therefore substantially equal to twice the diameter of the pulse, on which the The appearance of the treatment is not identical to the rest of the surface. Similarly, if the edges of the treatment field are defined by a periodic pattern, the overlap zone reproduces this pattern and is still visible to the naked eye.

The object of the invention is to provide a process for ultra-short pulse laser treatment of a surface of a product, such as a stainless steel sheet, but not limited to it, making it invisible to the naked eye the junction zone of several successive optical fields which would be arranged so that, taken together, they make it possible to treat a larger portion of the surface (typically its entirety) than a single optical field could do. This process should have a good productivity so that it can be applied in an economically viable manner on an industrial scale to the treatment of large surface products.

A cet effet, l’invention a pour objet un procédé de réalisation d’un effet visuel d’irisation sur la surface d’une pièce, selon lequel on envoie sur ladite surface des faisceaux laser, à durée de puise inférieure à une nanoseconde, dans les champs optiques juxtaposés des systèmes de focalisation d’au moins deux dispositifs fixes, ou dans le champ d’au moins un dispositif mobile, le ou lesdits dispositifs comprenant chacun une source laser, un scanner et ledit système de focalisation, de façon à conférer à ladite surface sur la largeur dudit puise une structure sous forme de vaguelettes, et on réalise un balayage par le ou lesdits scanners de ladite surface par lesdits faisceaux laser selon une série de lignes successives dans une direction de défilement relatif de la pièce et du ou des scanners et une série de lignes se situant dans le prolongement l’une de l’autre selon une direction perpendiculaire à ladite direction de défilement relatif, et appartenant chacune au champ optique du ou des dispositifs, chaque ligne étant de largeur égale au diamètre dudit puise, caractérisé en ce que lesdits champs optiques de deux dispositifs fixes voisins ou les champs optiques correspondant à deux positions successives dudit dispositif mobile se recouvrent dans une zone de recouvrement d’une largeur comprise entre deux fois le diamètre du puise du faisceau laser et 2 cm, de sorte que deux lignes se situant dans le prolongement l’une de l’autre se recouvrent au niveau d’une jonction, et en ce que, entre deux séries de lignes successives dans une direction de défilement relatif de la pièce et du ou des scanners, lesdites zones de jonction sont disposées de façon aléatoire ou de façon périodique organisée selon un motif aléatoire avec une périodicité égale à au moins dix fois la valeur maximale du décalage entre les jonctions présentes sur deux lignes successives selon ladite direction de défilement relatif à l’intérieur de ladite zone de recouvrement des champs optiques.

Entre la réalisation du balayage selon deux lignes successives dans ladite direction de défilement relatif de la pièce et du ou des scanners, on peut modifier la polarisation du faisceau laser de façon à créer des vaguelettes d’orientations différentes sur lesdites deux lignes successives. et d’orientation commune deux à deux dans la direction perpendiculaire à la direction de défilement relatif.

La polarisation du faisceau laser peut être modifiée selon un motif périodique, ledit motif périodique s’étendant sur M lignes consécutives selon la direction de défilement relatif de la pièce et des scanners, M étant égal à au moins 2, de préférence à au moins 3.

Deux lignes successives selon la direction de défilement relatif de la pièce et des scanners peuvent avoir des angles de polarisation qui diffèrent d’au moins 20°.

Les polarisations de deux lignes de deux champs voisins situées dans le prolongement l’une de l’autre peuvent être identiques.

On peut réaliser ledit déplacement relatif de ladite surface de ladite pièce et des dispositifs émettant lesdits faisceaux laser en plaçant ladite pièce sur un support mobile.

On peut réaliser ledit déplacement relatif de ladite surface de ladite pièce et des dispositifs émettant ledit ou lesdits faisceaux laser en plaçant le ou les dispositif(s) émettant lesdits faisceaux laser sur un support mobile.

Ladite pièce peut être une tôle.

Said surface of said part can be three-dimensional, the distance between the focusing system and the surface of the part is then measured, and the focusing system is controlled, so that it guarantees that the diameter of the pulse and the fluence of the beam laser are substantially the same regardless of the effective distance between the focusing system and the part.

Said surface of said part can be three-dimensional, the distance between the focusing system and the surface of the part is then measured, and the relative position of said device and of said surface is controlled so that the distance between said focusing system and said surface remains the same during the implementation of the method.

Said part can be made of stainless steel.

The invention also relates to a device for imposing an iridescent appearance on the surface of a part by forming ripples on said surface by the pulse of a laser beam, said device being formed of at least two juxtaposed fixed unit devices or at least one mobile unit device, each comprising a laser source generating a laser beam with a pulse duration of less than 1 ns, an optical system for shaping the beam, a scanner which allows the pulse of the beam , after passing through a focusing system, to scan in the form of lines an optical field on the surface of the part, said optical fields of two juxtaposed unitary devices overlapping over a width comprised between twice the diameter of the pulse of the laser beam and 2cm,containing the junctions of two lines each made by a unitary device, and means for creating a relative movement in a given direction between said device and said part so as to carry out the treatment on at least part of the surface of said part, characterized in that said scanners of said unitary devices make it possible to arrange said junctions so that these form, taken together, a random pattern, or periodically organized according to a random pattern with a periodicity equal to at least ten times the maximum value of the shift between the junctions present on two successive lines along said direction of relative travel inside said optical field overlap zone.and means for creating a relative movement in a given direction between said device and said part so as to carry out the treatment on at least part of the surface of said part, characterized in that said scanners of said unitary devices make it possible to arrange said junctions so that these form, taken together, a random pattern, or in a periodic manner organized according to a random pattern with a periodicity equal to at least ten times the maximum value of the offset between the junctions present on two successive lines according to the said running direction relative to the interior of said overlap zone of the optical fields.and means for creating a relative movement in a given direction between said device and said part so as to carry out the treatment on at least part of the surface of said part, characterized in that said scanners of said unitary devices make it possible to arrange said junctions so that these form, taken together, a random pattern, or in a periodic manner organized according to a random pattern with a periodicity equal to at least ten times the maximum value of the offset between the junctions present on two successive lines according to the said running direction relative to the interior of said overlap zone of the optical fields.characterized in that said scanners of said unitary devices make it possible to arrange said junctions so that the latter form, taken together, a random pattern, or in a periodic manner organized according to a random pattern with a periodicity equal to at least ten times the maximum value of the offset between the junctions present on two successive lines in said direction of relative movement inside said overlap zone of the optical fields.characterized in that said scanners of said unitary devices make it possible to arrange said junctions so that the latter form, taken together, a random pattern, or in a periodic manner organized according to a random pattern with a periodicity equal to at least ten times the maximum value of the offset between the junctions present on two successive lines in said direction of relative movement inside said overlap zone of the optical fields.or in a periodic manner organized according to a random pattern with a periodicity equal to at least ten times the maximum value of the offset between the junctions present on two successive lines in said direction of relative travel inside said overlap zone of the optical fields.or in a periodic manner organized according to a random pattern with a periodicity equal to at least ten times the maximum value of the offset between the junctions present on two successive lines in said direction of relative travel inside said overlap zone of the optical fields.

The optical systems of said unitary devices may comprise an optical polarization system which confers a determined polarization to said beam, and means for varying this polarization so that, on said surface, two neighboring lines in the direction are produced with pulses of different polarizations .

Said unitary devices can make it possible to produce two neighboring lines with pulses whose polarizations differ by at least 20°.

Said unitary devices may comprise means for measuring the distance between the focusing system and the surface of the part connected to means for controlling the focusing system so that the latter maintains a constant pulse diameter and fluence on said surface. , regardless of said distance.

Said unitary devices may comprise means for measuring the distance between the focusing system and the surface of the part, connected to means for controlling the relative position of said device and of said surface of the focusing system making it possible to maintain the distance constant. between said focusing system and said surface.

Said means for creating a relative movement in a given direction between said device and said part can comprise a mobile support for the part.

Said means for creating a relative movement between said device and said part can comprise a mobile support for said unitary devices.

As will have been understood, the object of the invention is to make invisible or almost invisible the junctions between two lines of the surface facing each other, and produced by at least two juxtaposed fixed laser scanner devices, the respective laser scanners of which are both move along an axis called “slow axis”, the fields of said devices overlapping slightly to avoid the risk of non-processing, or under-processing, of these junction zones. To this end, the junction points of said lines facing each other, each formed by a laser scanner device, are arranged randomly (that is to say that the respective rapid axes of the devices are in the

extension of each other), if we consider two sets of lines which are successive in the direction of relative movement of the part and of the laser scanner devices, called "fast axis", which is substantially perpendicular to the slow axis.

Optionally, the at least two juxtaposed fixed devices can be replaced by a single mobile laser scanner device, which is moved along the fast axis to successively produce lines facing each other in the optical fields corresponding to two successive positions of the device. mobile, which is technically equivalent to the simultaneous use of several such juxtaposed fixed devices but increases the duration of the treatment.

In other words, the points of junction of the lines facing each other generated by two immediately neighboring laser scanner devices (or a laser scanner device which has, in the meantime, been moved) are not located on a straight line oriented substantially according to the slow axis, therefore substantially perpendicular to the scanning direction (fast axis) of the laser devices. They form a broken line of random shape, or of periodic shape but organized in a periodically repeating random pattern (which excludes a regular periodic pattern such as a sinusoid), which remains contained within the area covering the fields of the two laser scanner devices, and whose general orientation is substantially perpendicular to said scanning direction. The junctions between the optical fields of two sets of successive lines, according to this general orientation, do not therefore form, taken together, a linear pattern, and this pattern is less visible to the naked eye than if it constituted a substantially straight line. . This pattern is also preferably not a periodic pattern with a short period which might also be visible to the naked eye. If the pattern is periodic, it is preferable that the length of the period is not less than ten times the maximum value of the offset between the junctions present on two successive lines. a short-period periodic pattern that might also be visible to the naked eye. If the pattern is periodic, it is preferable that the length of the period is not less than ten times the maximum value of the offset between the junctions present on two successive lines. a short-period periodic pattern that might also be visible to the naked eye. If the pattern is periodic, it is preferable that the length of the period is not less than ten times the maximum value of the offset between the junctions present on two successive lines.

La largeur de la zone dans laquelle lesdites jonctions sont présentes ne doit, de préférence, pas excéder 2 cm. Si cette largeur est trop réduite (inférieure à deux fois le diamètre du puise), on risque d’avoir une ligne brisée trop proche d’une ligne droite pour que le risque d’une visibilité des jonctions soit écarté. Si on a une largeur trop élevée, on réduit le champ optique utile des dispositifs, et on détériore ainsi la productivité de l’installation.

Bien entendu, si pour traiter l’ensemble de la surface de la pièce on a besoin de plus de deux dispositifs à laser scanner, l’invention s’applique, de proche en proche, pour tous les couples de dispositifs voisins.

Ce procédé et le dispositif associé peuvent aussi être utilisés avantageusement en conjonction avec un procédé et un dispositif associé destinés à supprimer, ou au

moins à très fortement atténuer, les problèmes liés à la directionnalité excessive de la vision de l’irisation de la surface d’un acier inoxydable traité par un dispositif comprenant un laser scanner. Selon ce procédé, on impose une polarisation différente de la lumière émise par le laser pour la formation des LIPPS de deux lignes disposées consécutivement selon la direction de déplacement relatif de la pièce et des dispositifs à laser scanner. L’utilisation de trois polarisations différentes au moins, appliquées à une série d’au moins trois lignes consécutives, est conseillée pour obtenir l’effet recherché.

L’invention sera mieux comprise à la lecture de la description qui suit, donnée en référence aux figures annexées suivantes :

La figure 1 qui montre la surface d’une tôle sur laquelle on a exécuté un traitement laser d’irisation selon l’invention au moyen de deux dispositifs à laser contigus, formant des lignes situées dans le prolongement l’une de l’autre selon les axes rapides des dispositifs, et avec des zones de recouvrement entre deux lignes disposées selon la variante préférée de l’invention, à savoir selon une ligne brisée aléatoire dont l’orientation générale est celle de l’axe lent, et non selon une ligne droite qui correspondrait sensiblement à l’axe lent, ou selon une ligne brisée périodique orientée généralement selon l’axe lent ;

La figure 2 qui montre le schéma de principe d’un dispositif selon l’invention, permettant la mise en oeuvre du procédé selon l’invention dans le champ optique d’un dispositif de traitement laser, et permettant également de rendre l’observation de l’irisation de la surface de la tôle indépendante de l’angle d’observation selon une variante préférée de l’invention ;

La figure 3 qui montre la surface d’une tôle résultant de la mise en oeuvre de la variante préférée du procédé selon l’invention, améliorant le procédé utilisé dans le cas de la figure 1.

Le but de l’invention est donc de réaliser un traitement dont les défauts ne seraient pas détectés facilement par l’œil humain, qui repère rapidement ce qui est linéaire, et aussi ce qui est périodique selon une brève période. Dans ce cas, si l’on considère que le traitement optimal de la surface de la tôle 1 nécessite N passages successifs d’un laser sur une même ligne correspondant à l’axe rapide d’un champ optique donné pour injecter une quantité d’énergie nécessaire et suffisante pour obtenir les vaguelettes désirées, le décalage aléatoire, par rapport à l’axe lent, des N lignes superposées est identique d’un passage du laser à l’autre.

La figure 1 schématise une telle configuration, réalisée sur une tôle 1 . On y voit que, pour des séries de deux passages (bandes de scan) du scanner 13 correspondant à

deux champs de scanners 13 voisins situés dans le prolongement l’un de l’autre et se superposant légèrement pour éviter la présence de zones non traitées sur la surface de la tôle 1 , les points de jonction 2 des champs optiques respectifs des deux séries 3, 4 de lignes réalisées respectivement par l’un des deux scanners sont décalés d’une façon non linéaire, mais aléatoire, entre deux lignes 5, 6, ou deux séries de N lignes superposées, disposées successivement selon l’axe lent 7 qui est la direction relative de progression de la tôle 1 et des scanners à laser 13. Autrement dit, les jonctions 2 respectives de deux lignes se faisant face et appartenant chacune à une série 3, 4 ne forment pas, prises ensemble, une droite ou un motif périodique à courte période, mais une ligne brisée aléatoirement qui est moins aisément discernable que ne le seraient une ligne droite ou une ligne brisée dans laquelle les décalages des jonctions 2 seraient périodiques avec une courte période.

Il est à noter qu’entre deux lignes successives 5, 6 réalisées dans le même champ optique et, donc, décalées dans la direction de progression 7 (autrement dit l’axe lent) des scanners à laser 13, ou dans la direction de progression de la tôle 1 si c’est elle qui est mobile selon l’axe lent alors que les scanners 13 sont fixes, ce problème ne se pose généralement pas avec la même intensité, sauf si le recouvrement entre les lignes de deux séries 3, 4 se succédant selon l’axe lent 7 est franchement mauvais.

C’est, en fait, le déplacement relatif relativement lent de la tôle 1 et des scanners 13 qu’il faut considérer, et que montre la flèche 7 qui définit l’axe lent.

En effet, comme on l’a dit, les différentes lignes 5, 6 de chaque groupe 3, 4 ont des largeurs sensiblement égales au diamètre du puise, soit de 30-40 pm environ, généralement. Pour assurer qu’il ne subsiste pas, sur la surface de la tôle, de zones non traitées entre deux lignes 5, 6 successives d’un même groupe 3, 4 selon l’axe lent 7, il est possible de régler le galvo du scanner et/ou le dispositif de déplacement de la tôle pour que deux lignes 5, 6 successives selon l’axe lent 7 se chevauchent.

Autrement dit, les lignes 5, 6 d’un même groupe 3, 4 sont formées après un décalage des positions relatives des puises de chaque scanner 13 et de la tôle 1 qui est légèrement inférieur au diamètre des puises. Il peut donc bien y avoir un double traitement de la surface de la tôle 1 dans les zones de chevauchement des lignes 5, 6, mais comme le décalage des lignes 5, 6 selon l’axe lent 7 est maîtrisable avec une bonne précision, nettement meilleure que la précision du recouvrement de champs optiques voisins selon l’axe rapide, la largeur de ces zones, si elles existent, est de toute façon suffisamment faible (quelques pm) pour que le double traitement ne se traduise pas visuellement par une perturbation de l’effet irisé par rapport à ce que l’on obtient sur le restant de la surface de la tôle 1. On peut aussi faire le choix de, délibérément, ne pas prévoir de recouvrement entre deux lignes 5, 6 successives selon l’axe lent 7, mais de viser un décalage très réduit, de l’ordre de quelques pm, en tout cas suffisamment réduit pour qu’il ne provoque pas l’existence de lignes non traitées qui seraient visibles à l’œil nu selon la direction perpendiculaire à l’axe lent 7.

It should be understood that, in FIG. 1, each series of lines 3, 4 located in the extension of one another and meeting at the level of a junction 2 is itself made up of the superposition of N lines superimposed, with, for example, N=3. The number of superimposed lines for a given optical field depends on the quantity of energy which it is necessary to bring to the surface of the sheet 1 to obtain the desired wavelet configuration , responsible for the iridescence of the surface. The higher this quantity, the higher the number of lines, for the same energy provided by each pass of the laser.

As far as possible, this configuration has a structure of the LSFL type, which has been seen to be the most suitable for providing this iridescence under conditions which are, however, dependent on the viewing angle. The energy brought along a given line must therefore be contained between a lower limit below which there would not be sufficiently pronounced wavelets, and an upper limit above which the probability of an excessive presence is increased too much. of Bumps. These limits are, of course, very dependent on multiple factors, in particular the precise material of the sheet 1, its surface condition, the energy supplied by the pulses during each passage of the laser over a given zone, etc.

Although this first approach makes it possible to substantially reduce the visibility of the overlapping of two successive fields, depending on the material used and/or the intended effect, due to the fact that the overlaps between fields (the junctions 2) are not arranged in a straight line corresponding to the slow axis 7, but according to a randomly broken line of which only the general orientation corresponds substantially to the slow axis 7, which follows the offsets between the overlaps, it may, however, prove to be insufficient to make the surface sufficiently homogeneous. In this case, it is possible to use the same approach, but changing the offset between the different passages of the laser. This makes it possible to further increase the random character of the positioning pattern of the overlaps with respect to the previous case. In other words, the broken line which joins the successive overlaps and constitutes said pattern has an even less obvious non-periodic or random character. But it is still necessary to ensure that the juxtaposed treatment fields have the same offsets as the first at each pass, because it is necessary to avoid the local accumulation of laser passes to have a

apparently homogeneous treatment, just as it is ideally necessary that any point of the surface receives the same quantity of energy according to the same distribution, the same number of pulses and passes.

The use of a pattern of superposition of the edges of random fields therefore makes it possible to distribute the points of heterogeneity, without these forming a straight line or a broken line with a short period, which would be too visible to the naked eye. . When the pattern they draw is identical for all the laser passes along the fast axis that have formed a given line, these overlapping points are locations where the heterogeneity is strong, because the discontinuity of the line is marked at each past.

As has been said, a certain periodicity of the pattern of the junctions 2 of the field edges is acceptable if it is carried out over a sufficient length along the slow axis 7, namely at least equal to ten times the maximum value of the shift between the junctions 2 present on two successive lines 5, 6 along the slow axis 7.

Performing the treatment in the form of lines oriented along the fast axis makes it possible to take advantage of the high repetition frequency of lasers with an ultra-short pulse duration to increase the productivity of the treatment. Thus, in a single scan of the line by the scanner along the fast axis, the line could be irradiated N times if the distance between two pulses of two neighboring fields is equal to the diameter of the pulse divided by N. This therefore makes it possible to erase the effect that small power fluctuations could have on the homogeneity of the surface. Thus, to obtain N successive irradiations of the same point on the surface of the sheet 1, it is not necessarily necessary to pass a laser beam 9 N times over said point, a single passage of the laser beam 9 may suffice.

This mode of action has, however, the drawback of forming areas of heterogeneity at the ends of the lines over distances equivalent to the diameter of a pulse (a few tens of micrometers).

As has been said, the iridescence effect obtained by treatment with a laser 8 with ultra-short pulses is linked to the spontaneous formation on the surface of the sheet 1 of a periodic structure having a behavior analogous to an optical grating. on the light reflecting off the surface. As discussed previously, the mechanism of formation of this structure in ripples distributed periodically on the treated surface has not yet been established by the scientific community.

However, it has been shown (see, for example, the document "Control Parameters In Pattern Formation Upon Femtosecond Laser Ablation", Olga Varlamova et al., Applied Surface Science 253 (2007) pp. 7932-7936), that the orientation of the wavelets was mainly related to the polarization of the laser beam irradiating the surface. Thus, on a stainless steel, the HSFLs have an orientation parallel to the polarization of the beam

incident whereas the LSFLs which then form, when a greater quantity of energy has been brought to the surface of the sheet, have an orientation perpendicular to the polarization of the incident beam. On other materials, the opposite effect can be observed, but this does not call into question the applicability of the invention to these materials.

Dans le cas d’un traitement laser par lignes, il en résulte donc qu’une surface traitée sans modification de la polarisation du faisceau laser 9 au cours de ses différents passages sur une ligne donnée de ladite surface présente, en fin de traitement, une structure constituée de lignes/vaguelettes toutes orientées dans la même direction. Ceci induit que l’effet“réseau optique” de la surface est également orienté.

En effet, l’irisation apparaît comme maximale si l’observation se fait dans une direction transverse à l’orientation des vaguelettes et elle diminue au fur à mesure que l’angle d’orientation de l’observation s’aligne avec la structure de la surface. Ainsi, une observation de la surface dans l’alignement des vaguelettes ne fait pas apparaître de couleur. Cela peut constituer un inconvénient pour le produit final car cela impose de bien choisir l’orientation des vaguelettes dès le traitement pour avoir un produit sur lequel l’irisation apparaît dans les conditions d’observation voulues. De plus, le produit final n’apparaît très pleinement coloré que selon une seule direction principale d’observation, pour une source lumineuse donnée.

La variante optimale de l’invention, objet des figures 2 et 3, permet de supprimer cet inconvénient. Si deux champs successifs, formant ensemble une même ligne sur toute la largeur de la tôle 1 (la direction perpendiculaire à l’axe lent 7), possèdent la même polarisation selon cette ligne, l’effet visuel d’un double traitement de la zone de jonction entre ces deux champs tend à être beaucoup moins marqué que si les deux champs ont des polarisations différentes, avec une différence d’angle de polarisation, différence qui est de préférence entre 20 et 90°.

D’autre part, avoir, selon la variante préférée de l’invention, des polarisations assurément suffisamment différentes entre deux lignes successives selon la direction relative de défilement 7 de la pièce et des dispositifs à laser scanner, supprime la directionnalité de l’observation de l’irisation. La conjugaison des phénomènes que l’on a décrits fait que l’irisation de la tôle 1 traitée paraît beaucoup plus uniforme, dans toutes les directions d’observation, que dans le cas où on n’a pas cette alternance de polarisation entre lignes voisines.

Le traitement est effectué“en lignes”, avec une distance séparant les centres des puises légèrement inférieure au diamètre du puise dans la direction de balayage rapide, pour qu’il n’y ait assurément pas de zones non traitées par le puise. La solution selon la variante préférée de l’invention consiste à alterner des lignes pour lesquelles l’orientation des vaguelettes est modifiée, d’une ligne à l’autre, par l’action d’un polariseur ou de tout autre type de dispositif optique polarisant, placé sur le chemin optique du faisceau laser 9.

Ainsi, soit le champ de traitement est réalisé avec un système automatique permettant de modifier la polarisation du faisceau incident entre chaque ligne, soit le champ de traitement est réalisé en un nombre de fois M égal à au moins deux, et de préférence à au moins trois, M correspondant, donc, au nombre d’orientations différentes que procurent aux vaguelettes les polarisations périodiquement successives du puise du faisceau laser qui les forme.

La figure 2 schématise une architecture typique d’une partie d’un dispositif unitaire permettant la mise en oeuvre du procédé selon l’invention, y compris dans la version préférée que l’on vient d’évoquer, pour traiter une partie d’une tôle 1 d’acier inoxydable sur un champ donné. Bien entendu, ce dispositif est commandé par des moyens automatisés, qui permettent de synchroniser les mouvements relatifs du support 15 de la tôle 1 et du faisceau laser 9, ainsi que de régler les paramètres du faisceau laser 9 et sa focalisation en fonction des besoins. La programmation de ces moyens automatisés ne relève que des compétences habituelles de l’homme du métier.

Le dispositif unitaire de la figure 2 comprend d’abord une source laser 8 d’un type classiquement connu pour la réalisation d’irisations de surfaces métalliques, donc, typiquement une source 8 générant un faisceau laser 9 pulsé de faible durée de puise (inférieure à une nanoseconde), le diamètre de chaque puise étant typiquement de l’ordre de 30 à 40 pm. L’énergie injectée sur la surface de l’acier inoxydable par le puise est à déterminer expérimentalement, de manière à générer sur la surface de la tôle 1 des vaguelettes LIPPS, de préférence de type LSFL et éviter la formation de bumps, a fortiori de pics, et la fréquence et la puissance du faisceau laser 9 doivent être choisies en conséquence selon les critères connus de l’homme du métier à cet effet et compte tenu des caractéristiques précises des autres éléments du dispositif et du matériau à traiter. Le faisceau laser 9 généré par la source 8 passe ensuite dans un système optique 10 de mise en forme du faisceau 9, qui, outre ses composants classiques 1 1 permettant de régler la forme et les dimensions du faisceau 9, comporte optionnellement, selon la variante préférée de l’invention, un élément optique polarisant 12 qui permet de conférer au faisceau 9 une polarisation choisie par l’opérateur ou les automatismes qui gèrent le dispositif.

Le faisceau laser 9 passe ensuite dans un dispositif de balayage (par exemple un scanner) 13 qui, comme il est connu, permet au faisceau 9 de balayer la surface de la tôle 1 selon une trajectoire rectiligne (l’axe rapide) dans un champ de traitement. En sortie du scanner 13, là encore de manière classique, on trouve un système de focalisation 14, tel qu’une lentille de focalisation, grâce auquel le faisceau laser 9 est focalisé en direction de la tôle 1.

In the example shown, the sheet 1 is carried by a mobile support 15 which makes it possible to move the sheet 1 in a plane, in the direction 7 (slow axis), or, possibly, in the three dimensions of space, by relative to the device for generating, polarizing and scanning the laser beam 9, so that the latter can treat the surface of the sheet 1 according to a new line of the treatment field of the device shown.

Preferably, before this processing of said new line, according to a variant of the invention, the result of which is illustrated in FIG. 3, the optical device 12 for polarizing the laser beam 9 has had its setting modified, so as to give the laser 9 a different polarization than it had when processing the previous row. At least two different angles of polarization, and preferably at least three, are capable of being obtained thanks to the optical polarization device 10, and alternate, preferably but not necessarily, periodically at each line change. A periodicity of the polarization pattern is not essential, it suffices, as has been said, that the angles of polarization of two lines 16, 17, 18, 16', 17', 18' which are neighbors along the slow axis 7 are different, preferably at least 20° and at most 90°. But a periodicity of the pattern, for example, as shown, with angles of polarization which repeat every three lines 16, 17, 18, 16', 17', 18' is preferred, insofar as a periodic programming of the change polarization is simpler than random programming, in particular as two lines 16, 16' or 17, 17' or 18, 18' belonging to two different fields and located in the extension of one another according to the fast axis and meeting at a junction 2 must have the same orientation of wavelets.

A succession of random polarizations within a given optical field, preferably nevertheless respecting the aforementioned minimum angular deviation of 20° to 90° would be acceptable.

According to the invention, the entire sheet metal processing device 1 comprises a plurality of unitary devices such as the one which has just been described, placed facing the sheet 1, and which are juxtaposed so that the fields respective treatment of two neighboring unit devices, that is to say the optical fields of the focusing systems 14 of their respective scanners 13, overlap slightly. This overlap is, typically, of the order of twice the size of the pulse, plus a position uncertainty which is linked to the period of supply of the laser 8 with pulses and to the scanning speed of the laser beam 9 according to the fast axis. We must verify experimentally that this

overlap is sufficient to ensure that no untreated areas remain on the sheet 1 at the end of the operation. Also, the lines 16, 16' or 17, 17' or 18, 18' generated by each of these fields must be continuous with each other on two neighboring fields, and the settings of the unit devices must be identical for two fields neighbors, in particular in terms of shape, size, power and angle of polarization at a time t of their respective laser beams 9, so that the treatment is homogeneous over the whole of a line of the width of the sheet 1, and that the alternation of the polarization angles of the laser beam 9 between two consecutive lines 16, 17, 18, 16', 17', 18' along the slow axis 7 is identical over the entire width of the sheet 1.

The control means of these unitary devices are, most typically, means common to all the unitary devices, so that they act in perfect synchronization with each other. They also control, preferably, the movements of the support 15 of the sheet 1, again for better synchronization of the respective movements of the sheet 1 and the laser beams 9 of the unitary devices.

Of course, the mobile support 15 could be replaced by a fixed support, and ensure the relative movement of the sheet 1 and the laser beams 9 of the unitary processing devices by placing them on a mobile support. The two variants can moreover be combined, in that the device according to the invention would comprise both a mobile support 15 for the sheet 1 and another mobile support for the unitary processing devices, the two supports being able to be actuated l one or the other, or both simultaneously, by the control device, according to the wishes of the user. In the case where there is only one processing device which is moved along the fast axis once it has processed part of the sheet 1 . In this case,

Also, the relative movement of the sheet 1 and the laser beams 9 of the unit devices along the slow axis 7 can be achieved using optical means integrated into the unit processing devices and acting on the locations of the fast axes of the movements of the laser beams 9 of these unit devices. These optical means replace, or are added to, the mechanical means for moving the movable support 15 of the sheet 1 and/or the movable support of the lasers 8 of the unitary processing devices.

Such purely optical means could suffice to process small-sized parts, but run the risk of not being sufficient to process relatively large-sized parts with sufficient precision. But it is possible to combine optical means and mechanical means by placing the mechanical means in a given position, then by carrying out the relative displacement of the laser beam 9 along the slow axis 7 by the optical means, over a sufficiently short distance "d". so that the precision of the relative displacement is sufficient, then by displacing the mechanical means by a distance equal to "d" to then continue the treatment of the sheet 1 by using the optical means to again carry out the relative displacement of the laser beams 9 and sheet 1 along the slow axis.

In the preferred variant of the invention, the number M therefore corresponds to the number of different orientations that it is desired to give to the ripples by ensuring a spacing M times greater than a conventional treatment and by shifting the lines by a spacing classic between each realization of the field. Figure 3 shows an example of how such a realization would look with M = 3.

The sheet 1 has on its surface a periodic succession of lines 16, 17, 18, 16', 17', 18' produced using two devices according to the invention which have made it possible to produce this periodic pattern of three kinds. such lines on two contiguous optical fields 19, 20, the lines 16, 17, 18 of a given field being in the extension of similar lines 16', 17', 18' made in the neighboring optical field.

Les lignes 16, 17, 18, 16’, 17’, 18’ du motif se distinguent les unes des autres par les effets des polarisations différentes que le dispositif de polarisation 12 a appliquées au faisceau laser 9 au moment de leur formation.

Comme on peut le voir sur la partie de la figure 3 qui représente une fraction agrandie de la surface de la tôle 1 , dans l’exemple représenté qui n’est pas limitatif, la polarisation conférée au faisceau laser 9 lors de la génération de la première ligne 16, 16’ du motif conduit à une orientation des vaguelettes dans la direction perpendiculaire à la direction relative de défilement 7 (axe lent) de la tôle 1 par rapport au dispositif de traitement laser. Puis, pour générer la deuxième ligne 17, 17’ du motif, on a modifié la polarisation du faisceau laser 9 de façon à obtenir une orientation des vaguelettes à 45° de l’orientation des vaguelettes de la première ligne 16, 16’. Enfin, pour générer la troisième ligne 18, 18’ du motif, on a modifié la polarisation du faisceau laser 9 de façon à obtenir une orientation des vaguelettes à 45° de l’orientation des vaguelettes de la deuxième ligne 17, 17’, donc à 90° de l’orientation des vaguelettes de la première ligne 16, 16’ : les vaguelettes de la troisième ligne 18, 18’ sont donc orientées parallèlement à la direction relative de défilement 7 de la tôle 1 par rapport au dispositif de traitement laser.

In the junction zone of two lines 16, 16', 17, 17', 18, 18', located in the extension of one another, one therefore injects onto the surface of the sheet 1 an energy greater than that which is injected onto the rest of the surface, just as in the basic variant of the invention previously described. This fact is marked by the existence of zones 2 having undergone an overprocessing and located at the junction of the neighboring fields, and whose precise situation inside the zone of overlapping of the fields is random in accordance with the invention. But the fact that in these junction zones 2, lines 16, 17, 18, 16', 17', 18' of each optical field which meet have been produced with the same polarization of the laser beam 9 attenuates even more clearly the alteration of the visual effect of iridescence of the surface of the sheet 1 which is observed in the absence of controlled polarization of the laser beam 9. The absence of continuity of the orientation of the wavelets from one optical field to another would tend to increase the visibility of the junction zone of the fields on a given line, by creating an area heterogeneity on the surface. It is simply necessary to ensure that the lines 16, 17, 18, 16', 17', 18' of the two neighboring fields which have been produced with identical polarizations are indeed in the extension of each other, but this precaution on the collinearity of lines 16, 16', 17, 17', 18, 18' of neighboring fields was also to be taken in the execution of the basic version of the method according to the invention, see FIG. 1, and the equipment used for this purpose can also be used in the context of this variant of the invention. It suffices to ensure that the changes in polarization of the laser beams 7 of the devices concerning each field take place with the same values ​​for the lines 16, 17, 18, 16', 17', 18' of the fields which join.

The use of M=2 different polarization orientations, phase-shifted, for example, by 90°, already makes it possible to have a visible iridescence effect in most directions of observation. However, the intensity resulting from the iridescence still varies quite significantly when observed at an angle of 45°, and it can be judged that the problem of the lack of directionality of the iridescence effect would still not be resolved satisfactorily. This is no longer visible as soon as M is greater than 2, preferably if the angles are separated by more than 20° between two successive lines 16, 17, 18, 16', 17', 18'.

Thus by carrying out a treatment with at least three distinct angles of polarization distributed between 0 and 90° and having, preferably, differences in polarization of at least 20° between two lines 16, 17, 18, 16', 17', 18' successive along the slow axis 7, experience shows that the iridescence of the surface is visible in all directions with a similar intensity. It is possible to use a number M of orientations greater than 3, but it is then necessary to ensure that the angles of polarization of two neighboring lines are sufficiently different from each other to obtain the desired absence of directionality of the iridescent effect.

It is however obvious that the distribution of the structure of the surface according to different orientations induces a reduction in the total intensity of the iridescence in comparison with a surface treated according to a single direction of polarization and observed according to the optimal angle (the angle transverse to the structure). There would therefore be a compromise to be found between the intensity of the visual iridescence effect perceived by the observer and the omnidirectional nature of this iridescence effect. But three directions of polarization (thus a periodicity of three lines of these directions, as shown in Figure 3) already represents, at least in the most common cases, such a good compromise.

Finally, to obtain the most homogeneous effect possible, it is recommended to alternate the orientations, preferably periodically, over the shortest possible distances. It will be preferred for M different orientations to periodically alternate a single line of each orientation, of equal width or, preferably (to ensure treatment of the entire surface of the sheet) slightly less than the diameter of the pulse.

It is possible to process sheets 1 whose flatness is not perfect by including in the processing device means for measuring the distance between the focusing system 14 and the sheet 1, and by coupling them to the control means of the focusing system 14, so that the latter guarantees that the diameter of the pulse and the fluence of the laser beam 9 are substantially the same regardless of the effective distance between the focusing system 14 and the sheet 1. As a variant, said means for measuring the distance between the focusing system 14 and the sheet 1 can be slaved to means for relative movement of the device according to the invention and of the sheet 1, making it possible to maintain constant the distance between the system focusing 14 and the surface of the sheet 1 for the duration of the treatment of the sheet 1.

It is also possible to envisage the application of the process to materials other than flat sheets (for example to shaped sheets, to bars, to tubes, to parts comprising three-dimensional surfaces in general), by adapting in consequently the means for relative movement of the lasers and the part to be treated, and/or the controls of the focusing means if differences in distance between the laser emitter and the surface have to be managed. In the case of parts with substantially cylindrical surfaces (bars and tubes of circular section, for example), one way of proceeding would be to place the laser devices on a fixed support and to provide, for the part, a support allowing it to be placed in rotation to scroll the surface of the part in the optical fields of the lasers.

Finally, it is recalled that if stainless steels are materials to which the invention is applicable in a privileged way, the other materials, metallic or non-metallic, on which the effect of iridescence of the surface by means of a laser treatment can to be obtained, are also concerned by the invention.

CLAIMS

1.- Method for producing a visual effect of iridescence on the surface of a part (1), according to which laser beams (9) are sent onto said surface, with a pulse duration of less than one nanosecond, in the juxtaposed optical fields of the focusing systems (14) of at least two fixed devices, or in the field of at least one mobile device, the said device or devices each comprising a laser source (8), a scanner (13) and the said focusing system (14), so as to give said surface over the width of said pulse a structure in the form of wavelets, and said scanner(s) (13) scan said surface by said laser beams (9) according to a series of lines (5, 6; 16, 17, 18, 16', 17',18') successive in a direction of relative travel (7) of the part (1) and of the scanner(s) (13) and a series of lines lying in the extension of one another in a direction perpendicular to said direction (7) of relative scrolling, and each belonging to the optical field of the device or devices, each line (5, 6; 16, 17, 18, 16', 17', 18') being of width equal to the diameter of said pulse, characterized in that said optical fields of two neighboring fixed devices or the optical fields corresponding to two successive positions of said mobile device overlap in an overlap zone of a width comprised between twice the diameter of the pulse of the laser beam (9) and 2 cm, so that two lines lying in the extension of each other overlap at a junction (2), and in that,between two series of lines (5, 6; 16, 17, 18, 16', 17', 18') successive in a direction of relative scrolling (7) of the part (1) and of the scanner(s) (13), said junction zones (2) are arranged randomly or periodically organized according to a random pattern with a periodicity equal to at least ten times the maximum value of the offset between the junctions (2) present on two successive lines (5, 6 16, 16', 17, 17', 18, 18') along said direction of relative travel (7) inside said optical field overlap zone.said junction zones (2) are arranged randomly or periodically organized according to a random pattern with a periodicity equal to at least ten times the maximum value of the offset between the junctions (2) present on two successive lines (5, 6 16, 16', 17, 17', 18, 18') along said direction of relative travel (7) inside said optical field overlap zone.said junction zones (2) are arranged randomly or periodically organized according to a random pattern with a periodicity equal to at least ten times the maximum value of the offset between the junctions (2) present on two successive lines (5, 6 16, 16', 17, 17', 18, 18') along said direction of relative travel (7) inside said optical field overlap zone.

2.- Method according to claim 1, characterized in that, between carrying out the scan along two successive lines (16, 17, 18, 16', 17', 18') in said direction of relative scrolling (7) of the part (1) and of the scanner(s) (13), the polarization of the laser beam (9) is modified so as to create wavelets of different orientations on said two lines (16, 17, 18, 16', 17', 18 ') successive and of common orientation two by two in the direction perpendicular to the direction of relative travel (7).

3.- Method according to claim 2, characterized in that the polarization of the laser beam (9) is modified according to a periodic pattern, said periodic pattern extending over M lines (16, 17, 18, 16', 17', 18') consecutive in the direction of relative travel (7) of the part and the scanners, M being equal to at least 2, preferably to at least 3.

4.- Method according to claim 2 or 3, characterized in that two lines (16, 17, 18, 16 ', 17', 18') successive in the direction of relative travel (7) of the part (1) and scanners (13) have polarization angles which differ by at least 20°.

5.- Method according to one of claims 2 to 4, characterized in that the polarizations of two lines (16, 17, 18, 16 ', 17', 18 ') of two neighboring fields located in the extension of one on the other are identical.

6.- Method according to one of claims 1 to 5, characterized in that said relative displacement of said surface of said part (1) and of the devices emitting said laser beams (9) is carried out by placing said part (1) on a mobile support (15).

7.- Method according to one of claims 1 to 6, characterized in that said relative displacement of said surface of said part (1) and of the devices emitting said laser beam(s) (9) is carried out by placing the device(s) emitting said laser beams (9) on a mobile support.

8.- Method according to one of claims 1 to 7, characterized in that said part (1) is a sheet.

9.- Method according to one of claims 1 to 7, characterized in that said surface of said part (1) is three-dimensional, in that the distance is measured between the focusing system (14) and the surface of the part (1), and in that the focusing system (14) is controlled so that the latter guarantees that the diameter of the pulse and the fluence of the laser beam (9) are substantially the same regardless of the effective distance between the focusing system (14) and the part (1).

10.- Method according to one of claims 1 to 7, characterized in that said surface of said part (1) is three-dimensional, in that the distance between the focusing system (14) and the surface of the piece (1), and in that the relative position of said device and of said surface is controlled so that the distance between said focusing system (14) and said surface remains identical during the implementation of the method.

1 1 .- Method according to one of claims 1 to 10, characterized in that said part (1) is made of stainless steel.

12.- Device for imposing an iridescent appearance on the surface of a part (1) by forming ripples on said surface by the pulse of a laser beam (9), said device being formed of at least two juxtaposed fixed unitary devices or at least one mobile unitary device, each comprising a laser source (8) generating a laser beam (9) with a pulse duration of less than 1 ns, an optical shaping system (10)

of the beam (9), a scanner (13) which allows the pulse of the beam (9), after it has passed through a focusing system (14), to scan in the form of lines (5, 6; 16, 17, 18, 16', 17', 18') an optical field on the surface of the part (1), said optical fields of two juxtaposed unitary devices overlapping over a width comprised between twice the diameter of the pulse of the laser beam (9) and 2 cm, containing the junctions (2) of two lines (5, 6; 16, 17, 18, 16', 17', 18') each produced by a unitary device, and means for creating a relative movement in one direction (7) given between said device and said part (1) so as to carry out the treatment on at least part of the surface of said part (1),characterized in that said scanners of said unitary devices make it possible to arrange said junctions (2) so that the latter form, taken together, a random pattern, or in a periodic manner organized according to a random pattern with a periodicity equal to at least ten times the maximum value of the offset between the junctions (2) present on two successive lines (5, 6; 16, 17, 18, 16', 17', 18') according to the said direction of relative scrolling (7) inside the said optical field overlap zone.18') along said direction of relative travel (7) inside said optical field overlap zone.18') along said direction of relative travel (7) inside said optical field overlap zone.

13.- Device according to claim 12, characterized in that the optical systems (10) of said unitary devices comprise an optical polarization system (12) which confers a determined polarization to said beam (9), and means for varying this polarization so that, on said surface, two lines (16, 17, 18, 16', 17', 18') adjacent in direction (7) are produced with pulses of different polarizations.

14.- Device according to claim 13, characterized in that said unit devices make it possible to produce two lines (16, 17, 18, 16', 17', 18') adjacent to pulses whose polarizations differ by at least 20 °.

15.- Device according to one of claims 12 to 14, characterized in that said unit devices comprise means for measuring the distance between the focusing system (14) and the surface of the part (1) connected to means control of the focusing system (14) so ​​that the latter maintains a constant pulse diameter and fluence on said surface, whatever said distance.

16.- Device according to one of claims 12 to 14, characterized in that said unit devices comprise means for measuring the distance between the focusing system (14) and the surface of the workpiece (1), and in that that these measuring means are connected to means for controlling the relative position of said device and of said surface making it possible to maintain constant the distance between said focusing system (14) and said surface.

17.- Device according to one of claims 12 to 16, characterized in that said means for creating relative movement in a given direction (7) between said device and said part (1) comprise a movable support (15) for the piece (1 ).

18.- Device according to one of claims 12 to 17, characterized in that said means for creating a relative movement between said device and said part (1) comprise a movable support for said unitary devices.