FR2772650A1 - Photon aluminum facade building cleansing system - Google Patents

Abstract

The pulse duration is matched to the minimum thickness of the pollutant layer and so as to minimize heat dissipation preventing damage to the facade.

Description

 The invention relates, in general, to the cleaning of supports covered with a layer of material to be removed.

 It aims, in particular, to make it possible to perform such cleaning effectively by scanning the support with a photonic beam, in particular of the laser type, adapted to remove the layer covering the support.

To this end, it proposes a cleaning process in which a layer of material covering a support is eliminated by scanning said support with a photonic beam, characterized in that a beam formed by pulsed radiation is used as said photonic beam.
having a pulse duration less than a minimum duration of propagation of heat through said layer of material, fixed by the minimum thickness of said layer on said support and by the speed of diffusion of heat in said material;
having, in the photonic spectrum, a wavelength for which the absorption by the support material and the absorption, greater, by said material, have a difference which is at a maximum; and
- having, over the duration of a pulse, a power density per unit of illuminated surface below the threshold of damage to the support at said wavelength and greater than the less important threshold of vaporization of said material at said length wave.

 The cleaning is thus carried out in a photoablation regime, that is to say in a regime where the illumination by the photonic beam leads to an instantaneous evaporation of the material to be removed, The emission of vapor generating a high local pressure at the support material layer interface, this pressure inducing an ejection of the material layer in the form of aerosols and particles.

 The choice of a pulse duration (conventionally measured at mid-height of the power timing diagram) less than the duration of propagation of the heat through the layer of material to be removed makes it possible to avoid damaging the support by a contribution of excessive heat, the violent heat produced due to the vaporization of the material to be removed, never completely reaching the support since the pulse, and therefore the production of heat, ends before the time necessary for the heat produced to pass through the layer of material. Taking into account the propagation time corresponding to the minimum thickness of the layer of material to be removed guarantees that at any location the pulse will be terminated sufficiently early.

 If certain areas of the support are covered with a layer of material to be removed relatively thicker than the minimum thickness, this layer can be removed with several pulses, i.e. it will be enough for the operator to sweep several times the area in question.

Once the layer of material to be removed is removed,
The application of the photon beam on the support does not risk damaging it since it is below its damage threshold.

 The choice of a wavelength such that there is a maximum difference between the absorption by the support material and the absorption by the material to be removed makes it possible to combine in the best conditions the contradictory requirements that are the respect of the support and the elimination of the layer of material to be removed, since one takes into account the combination of the reaction of the support and that of the layer of material to be removed, unlike some previous processes in which the parameter which constitutes the wavelength is neglected or then taken into account with respect to the only material of the layer of material to be removed.

 Taking into account the damage threshold of the support and the vaporization threshold of the material to be removed considered at the wavelength of the photon beam, offers the advantage of excellent accuracy (these two thresholds depend on the length wave) to minimize the risk of damage to the support or, conversely, non-vaporization of the material to be removed, while the determination of the aforementioned thresholds in terms of power density offers the advantage to take into account the speed of energy supply, contrary to a determination that would be made only in terms of energy density.

 It will be noted in this regard that the rate of energy supply is an important parameter for implementing the cleaning process with good performance, since as regards the layer of material to be removed, it is not only necessary to perform a sufficient supply of energy per unit area, but also that this supply takes place quickly enough so that the evacuation of heat out of the material to be removed, in particular by conduction, does not prevent its vaporization; and with regard to the support, it is necessary that the evacuation of energy, by conduction and, if it is a metal, especially by reflection, is sufficient so that the absorbed energy does not modify the support characteristics.

According to preferred characteristics due to the quality of the results obtained both with regard to the elimination of the layer to be removed and the respect for the support:
said radiation has a pulse duration less than 200 ns with said layer of material which is a deposit of pollutant of organic nature, and preferably said pulse duration is between 30 and 130 ns;
said radiation has a wavelength between 9 and li ptm with said support which is made of metal and said layer of material which is a deposit of organic pollutant, and preferably said wavelength is 10.6 pm ; and or
the power density per unit of surface illuminated over the duration of a pulse is between 4 and 7 MW / cm 2 with said support which is made of colorless anodized aluminum and said layer of material which is a deposit of organic pollutant; or then between 3 and 10 MW / cm2 with said support which is made of colored anodized aluminum and said layer of material which is a deposit of organic pollutant; and preferably the power density is 5 MW / cm2.

 It will be noted that the above wavelength range is particularly advantageous from the point of view of safety for the human and animal eye, the radiation at these wavelengths not passing through the glass or plastic panes.

 Thus, in practice, it is sufficient, to ensure the safety of persons assisting with cleaning, that they are behind a window or a bay window while simple glasses of glass or even of plastic material are suitable for ensuring the safety of the operator.

 The use of the method according to the invention is particularly advantageous for cleaning building elements, and in particular interior or exterior elements forming part of a facade wall, especially when these elements are made of anodized aluminum.

 This type of material, widely used in the facades of buildings, is in fact particularly advantageous to clean with a photonic beam in accordance with the invention because it is difficult, with conventional chemical and mechanical means, to remove the layer of pollutant s '' being deposited over time without removing the anodized part of the aluminum support (it is the anodized part which prevents corrosion of the support), while the use of a photon beam conforming to the above parameters makes it possible to remove very effectively the pollutant layer without producing the slightest attack from the support which hardly heats up moreover (we manage to obtain that 98% of the beam energy is absorbed by the pollutant layer and only 2% by the support) .

 The invention also relates, in a second aspect, to a laser source which is particularly suitable for implementing the method which has just been described.

 This laser source, comprising an envelope inside which are arranged an anode and a cathode to generate a laser effect by the stimulated emission of photons by molecules of a gas found in the envelope when an electric discharge suitable is produced between the anode and the cathode, is characterized in that a heat exchanger is also arranged inside said envelope.

 Thanks to the fact that the heat exchanger is arranged directly inside the envelope, and not outside of it, it is possible to maintain in a particularly simple and effective way the temperature of the gas in the range of values required to produce the photon beam having the desired parameters, in particular as regards its energy per pulse.

 Preferably, in order to automatically maintain the temperature at the appropriate value, a temperature probe is also placed inside said envelope, of the heat transfer fluid circulating in said exchanger and in an external regulation unit connected to said temperature probe. temperature in order to regulate the internal temperature of said envelope within a predetermined range.

 In particular, when a wavelength of the order of 10.6 Am is desired, said gas is CO2 and said exchanger is adapted to regulate said internal temperature at 27 + 2 "C.

 The invention also relates, in a third aspect, to a laser source which is particularly suitable for implementing the method described above.

 This source, comprising an envelope inside which are arranged an anode and a cathode to generate a laser effect by the stimulated emission of photons by molecules of a gas found in the envelope when an appropriate electrical discharge is produced between the anode and the cathode, is characterized in that it comprises a fan also arranged inside said envelope for circulating therein the gaseous medium which it contains.

This fan makes it possible in particular to renew the gaseous medium located between the anode and the cathode rapidly so that it is possible to obtain a particularly high pulse frequency, for example 200
Hz, while maintaining the desired parameters of the beam produced, in particular the energy per pulse.

 Thanks to such a high frequency, the cleaning speed (surface cleaned per unit of time) can be relatively high.

 According to preferred characteristics, making it possible to renew the gaseous medium between the electrodes in a particularly convenient and efficient manner, said fan extends over the same length as the anode and the cathode and comprises a casing having an intake opening, an opening discharge facing the space between the anode and the cathode and a rotor for circulating the gaseous medium in the casing, in the direction from said inlet opening to said discharge opening, in order to put circulating the gaseous medium contained in the envelope so that there is a flow leaving the delivery opening and passing through the space between the anode and the cathode before heading towards the inlet opening .

 According to other preferred characteristics, the source further comprises a heat exchanger disposed inside said envelope so that said flow of gaseous medium passes through the exchanger between the anode and cathode on the one hand and said opening of admission on the other hand.

 There is thus a gaseous medium further having the right temperature.

 According to other preferred characteristics, in the path between the anode and cathode on the one hand and said intake opening on the other hand, there are an outlet mouth and an inlet mouth of gaseous medium in said envelope.

 The used gaseous medium is extracted periodically, for example once a week, by the outlet mouth while new gaseous medium is simultaneously introduced by the inlet mouth, which makes it possible to renew the gas content of the envelope in order that there is between the anode and the cathode a gaseous medium having an optimal composition.

 The invention also relates, in a fourth aspect, to a waveguide which is particularly suitable for implementing the method described above.

 This waveguide is characterized in that it is formed by a flexible tubular body internally covered with a reflective dielectric coating, with the hollow of said body which is filled with a gas very inert to ionization.

 Thanks to the filling with this gas, it is avoided that there occurs within the waveguide a breakdown (sudden ionization of the gas) which would lead to the destruction of the waveguide.

 Preferably, said gas fills said hollow at a pressure slightly higher than atmospheric pressure.

 One is thus sure that it is indeed the very inert gas which is present in the hollow of the waveguide, and in addition one avoids the penetration of dust in this hollow, also likely to produce breakdowns.

 Preferably, said gas is helium, this gas being the one with the highest breakdown voltage.

 The invention also relates, in a fifth aspect, to a head for applying a laser beam which is particularly suitable for implementing the method described above.

 This application head is characterized in that it comprises optical processing means so that said laser beam illuminates on the support to be cleaned an area of tapered shape, that is to say extending essentially in width while its thickness is small.

 With such a shape, we manage to clean in a single pass a relatively large width of the support.

 Preferably, for reasons of simplicity and convenience, said optical processing means comprise a cylindrical lens whose position corresponds to the working distance provided between said head and the support to be cleaned.

 The invention also relates, in a sixth aspect, to a head for applying a laser beam which is particularly suitable for implementing the method described above.

 This head is characterized in that it comprises means for blowing or aspirating the vapors and dust resulting from the vaporization by the laser beam of the material to be removed.

 This prevents dust from redepositing on the support which has just been cleaned or in its surroundings, and above all that vapors and dust do not come to get in the path of the photon beam, which would reduce their efficiency.

 Preferably, in order to provide a cleaning of as good quality as possible, said head comprises means for suctioning said vapors and dusts, provided with a filtration element for retaining said dusts.

 The invention also relates, in a seventh aspect, to a head for applying a laser beam which is particularly suitable for implementing the method described above.

 This head is characterized in that it comprises means for detecting the presence of a metal opposite said head, in order to prohibit the emission of a laser beam in the absence of metal.

 The emission of a laser beam can therefore only be carried out when the head is facing the metal support to be cleaned, which is particularly advantageous in terms of security or even energy savings.

 The invention also relates, in an eighth aspect, to a head for applying a laser beam which is particularly suitable for implementing the method as described above.

 This head is characterized in that it carries means of contact with the support to be cleaned in order to fix the working distance between the head and the support.

 It is thus certain that the illuminated area on the support has the intended surface, and therefore that the desired parameters are respected, in particular as regards the power or energy density per unit of illuminated surface.

 Preferably, for reasons of simplicity and ease of movement of the head, said contact means comprise at least one roller intended to be moved on said support.

 The invention finally relates, in a ninth aspect, to a device which is particularly suitable for implementing the method as set out above, characterized in that it comprises a laser source as set out above, a guide for waves as exposed above and / or an application head as exposed above.

The description of the invention will now be continued with the description of a preferred embodiment, given below by way of illustration and not limitation, with reference to the accompanying drawings. On these
- Figure 1 shows schematically a cleaning device according to the invention;
- Figure 2 is an elevation-sectional view taken transversely in the laser source forming part of the device shown in Figure 1;
- Figure 3 is another view in elevation-section, taken longitudinally and showing only the elements for producing the laser beam;
- Figure 4 is an enlarged detail of the upper electrodes, and more precisely of their end as seen on the left in Figure 3;
- Figure 5 shows the constitution of a variant of the waveguide forming part of the device illustrated in Figure 1;
- Figure 6 is a partial view of the support to be cleaned, taken around the area illuminated by the laser beam of the cleaning device;
- Figures 7 and 8 are respectively an elevational view and a plan view showing the optical treatment applied to the laser beam between the output of the waveguide and the support to be cleaned;
- Figure 9 is an elevation-sectional view showing the end fitting of the laser beam application head;
- Figure 10 is a schematic view of the laser beam application head which illustrates the triggering elements and maintaining the working distance vis-à-vis the support; and
- Figure 11 is a simplified electrical diagram of the triggering means.

 The cleaning device 1 illustrated in FIG. 1 is provided for removing a layer 2 of organic pollutant covering, in the form of a deposit which has formed over time, a support 3 of anodized aluminum.

 The device 1 comprises a head 4 for applying a laser beam 5 to the support 3, a waveguide 6, one of the ends of which is connected to the head 4, a laser source 7 to which the other end of the guide 6 which is used to transport the laser beam produced by the source 7 to the head 4, a power supply 8 from the source 7, a unit 9 for thermal regulation of the source 7 and a unit 10 for renewal of source 7 in a gaseous medium consisting of a mixture of carbon dioxide, nitrogen and helium.

 As can be seen in FIGS. 2 and 3, the source 7 comprises an envelope 11 having a mouth 12 for entering the gaseous medium, a mouth 13 for leaving this medium, a tangential fan 14, an anode 15, a cathode 16 , pre-ionization electrodes 17 and a regulating heat exchanger 18.

The envelope It comprises a tube 19 with circular section, here in
Pyrex with a diameter of 215 mm and a length of 300 mm, and two metal supports 20A and 20B (Figure 3) which close tightly
The space inside the tube 19, respectively on the side that can be seen on the left and the side that can be seen on the right in FIG. 3. The various components placed inside the envelope 11 are fixed to the supports 20A and 20B.

 The support 20A also carries a mirror 20C for outputting the laser beam from the source 7, the reflection coefficient of which is around 80% at the wavelength of 10.6 clam, the support 20B bearing him a 20D mirror.

 The mirrors 20C and 20D are positioned opposite the space existing between the anode 15 and the cathode 16, in order to form an optical cavity in which the laser effect is generated by the stimulated emission of photons at 10.6 Fm by certain CO2 molecules located between the anode 15 and the cathode 16 when an appropriate electrical discharge is produced between them.

 In the example illustrated, the output mirror 20C is a blade with flat faces made of zinc selenide and the mirror 20D is spherical and made of gold-treated copper.

The radius of curvature of the mirror 20D is large so that it produces a beam whose angular divergence is small. The distance between the mirrors 20C and 20D is of the order of 250 mm, the mirror 20C is fixed and a micrometric positioning system is provided for the mirror 20D in order to match the optical cavity that it forms with the mirror 20C, on the wavelength of 10.6 Cim.

 Ports 12 and 13 are normally closed. When it is necessary to renew the gaseous medium contained in the enclosure 11, they are each connected to the unit 10 and are used respectively for injection into new gaseous medium in the space internal to the envelope 11 and for extraction of the used medium contained in this envelope.

 The tangential fan 14 extends over the same length as the electrodes 15 and 16. It comprises a casing 21 having an intake opening 22 facing the mouth 12 and a discharge opening 23 facing the space between the anode 15 and the cathode 16 as well as a rotor 24 for circulating the gaseous medium in the casing, in the direction going from the opening 22 to the opening 23.

 We see that the fan 14 circulates the gaseous medium contained in the envelope II so that there is a flow, illustrated by arrows in Figures 2 and 3, leaving the discharge opening 23, passing through the space between the electrodes 15 and 16 then by the regulating exchanger 18 before heading towards the intake opening 22.

 The flow created by the fan 14, combined with the presence and arrangement of the exchanger 18, makes it possible to have permanently, between the electrodes 15 and 16, a gaseous medium whose composition and temperature are suitable for production of the desired laser beam.

 The heat transfer fluid coming from the regulating unit 9 enters the exchanger 18 through the orifice 25, this fluid circulates in this exchanger while transferring its frigories or its calories to the gaseous medium, and it leaves the exchanger 18 by the orifice 26 to join the regulation unit 9.

 A temperature probe 26 ′ is provided in the space internal to the envelope 11 and connected to the regulating unit 9 which reacts accordingly to heat or cool the heat transfer fluid, the temperature targeted in the envelope being here 27 t 2 "C, optimal range for the emission of a CO2 laser beam.

 The regulation unit 9 comprises, for heating or cooling the thermal fluid, a Peltier effect element, so that the unit 9 is compact and requires only electrical energy for its operation.

 The anode 15 has the general shape of a flat plate, here a length of 240 mm, a width of 30 mm and a height of 8 mm.

 The cathode 16 is also in the form of a flat plate, the same length as the anode 15, but narrower (here, 10 mm) while its face opposite the anode 15 has teeth 27 regularly spaced , as seen more particularly in Figure 4.

 In each space between two teeth 27, is housed a pre-ionization electrode 17, surrounded by a tube 28 of silica.

 In the example illustrated, there are 105 teeth 27 and 104 sets of electrode 17 - tube 28 of silica. These tubes have an internal diameter of 1 mm and an external diameter of 2 mm, their length being 40 mm. The width of each electrode 17 is 45 mm and the difference between two successive electrodes 17 is 2.3 mm. The teeth 27 have a width of 0.3 mm, a height of 1 mm, and are separated from each other by a distance of 2 mm.

 The electrodes 17 are held together by two longitudinal metal rods, not shown, the mounting of the electrodes 17 on these rods being similar to that of the bars of a ladder on the uprights thereof.

 In addition to their mechanical function of holding the electrodes, the aforementioned rods make it possible to electrically connect the pre-ionization electrodes 17 to each other.

 Three spacers (not shown) are used to maintain the relative positioning of the anode 15, the cathode 16 and the assembly of pre-ionization electrodes 17, silica tubes 28 and longitudinal metal rods.

 The spacers are of course made of a very insulating material such as machinable glass ceramic.

 It will be noted that for reasons of ease of reading, the proportions of the electrodes 15, 16 and 17 are not observed in the drawings.

 Thus, the actual length of the silica tubes 28 of the example described, which is 40 mm, is greater than the width of the anode 15 (which is 30 mm); and the spacing between the cathode 16 and the anode 15 is 8 to 10 mm, that is to say that it corresponds to the width of the cathode 16.

 We will now explain the operation of the laser source 7.

 It is a source of the CO2 laser type TEA (Transverse Excitation at Atmospheric pressure), in which the gaseous medium is at atmospheric pressure, and is excited in pulse mode by electric discharges of several tens of kV between l anode 15 and cathode 16 which are parallel to the axis of the laser beam to be produced.

 The electrodes 15 and 16 are used to generate volumetric discharges uniformly distributed in the inter-electrode space, allowing the excitation (electronic pumping) of the CO2 molecules, which pass from the fundamental vibrational level 000 to the level 001, which gives rise to following the stimulated emission of laser radiation of wavelength 10.6 zm.

 Nitrogen acts as a buffer gas: it resonantly transfers its energy to the CO2 molecules after being excited by the electric discharge.

 The role of helium atoms is twofold: on the one hand to homogenize the electric discharge by elastic collisions with the electrons, on the other hand to deactivate the lower level of the laser transition (intermediate level 010) of the CO2 molecules.

 Before producing the above-mentioned electric discharge between the electrodes 15 and 16 (main discharge), a high voltage is applied, briefly, between the cathode 16 and the pre-ionization electrodes 17, and this results from the tubes silica 28 intense emission of ultraviolet light.

 The ultraviolet photons generated will ionize, for the most part, the impurities present in the gaseous medium contained in the enclosure 11, which produces a release of free electrons which, during the main discharge, will be accelerated by the electric field created between cathode 16 and anode 15.

 These accelerated free electrons will in turn generate secondary electrons by interaction with the neutrals of the surrounding gaseous medium (electronic avalanche).

 This gives a volumetric discharge uniformly distributed in the space between the anode 15 and the cathode 16, which optimizes the pumping of the CO2 molecules, essentially by means of the nitrogen molecules (buffer gas).

 The renewal of the gaseous medium (amplifying medium), carried out as explained above, is necessary because of the decomposition, during the discharges, of the molecules of CO2 into molecules of CO and O2 capable of creating arcs in the space inter- electrodes.

 The renewal of the gases between two discharges by a transverse flow using the tangential fan 14 makes it possible to reach a frequency of laser shots of 200 Hz.

 The power supply 8 is of the conventional type which comprises a high voltage generator for charging a capacity and a thyratron for closing at the desired frequency a circuit making it possible to apply the high voltage to the electrodes of the source 7 and to discharge the capacity therein. .

It will be observed that the assembly constituted by the laser source 7,
The power supply 8 and the control unit 9 is particularly compact and light, which makes it easily transportable.

 In practice, in a site for cleaning a support 3 by means of the device 1, the assembly formed by the elements 7 to 9 forms a block which is placed on the ground, and the operator holds the head 4 in hand. to walk it in front of the support 3 to scan the entire surface to be cleaned with the laser beam 5.

 The waveguide 6 which is used to transport the laser beam from the source 7 to the head 4, is formed by a

 Filling the hollow of the waveguide with slight overpressure guarantees the presence of helium and also offers the advantage of avoiding the entry of dust into the hollow.

 We will now describe, using FIG. 5, a variant in which the flexible waveguide 6 is replaced by an articulated waveguide 6 'and the head 4 by a head 4'.

 The guide 6 'is formed by an articulated arm provided with a mirror in each elbow, the guide 6' being more precisely constituted here by seven tubular steel segments 29A to 29G and by seven flat copper mirrors covered with gold 30A to 30G.

 We see that the mirror 30A is centered on the axis of rotation between the segments 29A and 29B, so that whatever the relative position of these segments, the laser beam 31 exiting the source 7 propagates in the segments 29A then 29B to the mirror 30A which then reflects it towards the mirror 30B, which is centered on the axis of rotation between the segments 29B and 29C, and so on for the successive segments and mirrors, until the laser beam reaches the head 4 '.

 The head 4 ′ is provided for transforming the laser beam leaving the waveguide 6 ′ into a laser beam 5 capable of appropriately illuminating the support 3 coated with the layer of pollutant 2, in particular as regards the geometric shape of the illuminated area, this shape should be such that the power density per unit of illuminated area has the required value, here 5 MW / cm2, and that the illuminated area is sufficiently large so that at each passage of the head a surface significant of support 3 has been cleaned.

 As can be seen in FIG. 6, in the present example, the illuminated zone 32 has a tapered shape, that is to say that it extends essentially in width while its thickness is particularly small. An illuminated surface of the order of 4 mm2 being sought here, the width of the area 32 is 30 mm and its average thickness is 0.13 mm.

 We will now explain, in support of FIGS. 7 and 8, how the head 4 'transforms the beam 33 leaving the waveguide 6' to give the beam 5 capable of illuminating the area 32 on the support 3.

 It will first of all be noted that the beam 33 has the same or almost the same optical properties as the beam 31 leaving the source 7, these properties being preserved during successive reflections on the mirrors 30A to 30G. In particular, the beam 33 is a slightly divergent rectangular beam.

 The head 4 ′ therefore provides a simple cylindrical lens 34, for example made of zinc selenium, to effect the beam 33 - beam 5 transformation, the focal distance of the lens 34 being chosen as a function of the working distance provided between the head 4 and the support to be cleaned.

 In the embodiment of FIG. 1 where there is not a mechanical waveguide but a flexible waveguide 6, the beam 31 leaving the source 7 loses in the guide 6 part of its geometric properties , the head 4 is similar to the head 4 ′ but also comprises, before the lens 34, a device for optical treatment of the beam leaving the waveguide 6, such as a polydiamond lens, so that the beam is of section rectangular and of slight divergence at the entry of lens 34.

 The head 4 or 4 ′, in addition to the means described above for transforming the beam leaving the waveguide into a beam 5, has a tip 35 shown in FIG. 9.

 The end piece 35 has a wall 36 forming a separation from the closed space in which the lens 34 is located, the wall 36 having a central opening closed by a window 37 made of zinc selenium allowing the beam 5 to pass through. without modifying the optical characteristics.

 After passing through the window 37, the bundle 5 propagates in an internal tube 38 disposed concentrically with the wall 36, comprising a cylindrical part of which one end is connected to the wall 36 while the other end is connected to a second part of the tubing 38, which is frustoconical.

 A tube 39 surrounds the wall 36 and the tube 38, so that there exists between the tubes 38 and 39, an annular chamber 40 supplied with compressed air by a mouth 41 located near the wall 36, the air injected by the mouth 41 crossing the chamber 40 until reaching its opposite end, where the air is ejected in the form of a conical jet centered around the beam 5, this jet propagating as shown by arrows in FIG. 9 (convergence up to support 3 then divergence after rebound on this support) and carrying with it the vapors and dust 42 resulting from the action of the laser beam 5 on the layer of pollutant 2.

 The conical air jet expelled by the head 35 thus makes it possible to prevent the vapors and dusts 42 from being redeposited on the surface of the support 3 and above all, that these gases and dusts do not absorb part of the laser light. , which would greatly reduce the effectiveness and efficiency of the cleaning performed.

 In a variant not shown, the nozzle 35 is replaced by a similar nozzle, but sucking the vapors and dust 42 rather than blowing them, a filtering element being provided in the suction system to recover the dust.

 FIG. 10 shows the head 4 in a more complete manner than FIG. 1: there is shown, very schematically, the elements with which it is provided to allow the triggering of the laser source and so that it remains at a distance of work desired with respect to the support 3 possibly coated with the layer of pollutant 2.

 To maintain the working distance, the head 4 carries two rollers 43 each mounted on a support 44 so that when the two rollers 43 are in contact with the support 3, the head 4 is exactly at the desired working distance .

 To scan the support 3 with the beam 5, the operator must simply move the head 4 while keeping the rollers 43 in contact with the support 3.

 In a variant not shown, the rollers 43 are replaced by simple sliding protuberances, for example covered at their end with a material with a low coefficient of friction, in particular Teflon.

 In other variants not shown, there is only one roller or only one protuberance and / or there is a casing avoiding a radial overshoot relative to the general size of the head.

 To allow the operator to trigger the operation of the laser source 7, in order to have the beam 5, the head 4 is provided with a push button 45 which the operator must keep pressed for there to be production of a laser beam.

 There is further provided a metal detecting coil 46, in order to prohibit the production of a laser beam when the head 4 is not facing a metal support.

 As can be seen in FIG. 11, the push button 45 and the coil 46 are connected to an electronic circuit 47 itself connected to the power supply 8, the circuit 47 allowing the power supply 8 to be switched on only if the push button 45 is pressed and if the coil 46 detects metal.

 Many variants are possible depending on the circumstances, and it is recalled in this regard that the invention is not limited to the examples described and shown.

Claims (27)

 1. A cleaning method in which a layer of material covering a support is removed by scanning said support with a photonic beam, characterized in that a beam formed by pulsed radiation is used as said photonic beam (5)  having a pulse duration less than a minimum duration of propagation of heat through said layer of material (2), fixed by the minimum thickness of said layer (2) on said support (3) and by the speed of heat diffusion in said material  - having, in the photonic spectrum, a wavelength for which the absorption by the material of the support (3) and the absorption, greater, by said material (2), have a difference which is at a maximum and  - having, over the duration of a pulse, a power density per unit of illuminated area below the threshold of damage to the support (3) at said wavelength and greater than the less important threshold of vaporization of said material (2) at said wavelength.  2. Method according to claim 1, characterized in that said radiation has a pulse duration less than 200 ns with said layer of material which is a deposit of organic pollutant.  3. Method according to claim 2, characterized in that said pulse duration is between 30 and 130 ns.  4. Method according to any one of claims 1 to 3, characterized in that said radiation has a wavelength between 9 and li um with said support which is made of metal and said layer of material which is a deposit of pollutant of organic nature.  5. Method according to claim 4, characterized in that said wavelength is 10.6 pm.  6. Method according to any one of claims 1 to 5, characterized in that the power density per unit of area illuminated over the duration of a pulse is between 4 and 7 MW / cm2 with said support which is made of aluminum colorless anodized and said layer of material which is a deposit of organic pollutant.  7. Method according to any one of claims 1 to 5, characterized in that the power density per unit of surface illuminated over the duration of a pulse is between 3 and 10 MW / cm2 with said support which is made of aluminum colored anodized and said layer of material which is an organic pollutant deposit.  8. Method according to any one of claims 6 or 7, characterized in that the power density per unit of area illuminated over the duration of a pulse is 5 MW / cm2.  9. Method according to any one of claims 1 to 8, characterized in that it is used to clean building elements.  10. Method according to claim 9, characterized in that said elements are part, inside or outside, of a building facade wall.  11. Device suitable for implementing the method according to any one of claims 1 to 10, comprising a laser source having an envelope (11) inside which are an anode (15) and a cathode (16 ) to generate a laser effect by the stimulated emission of photons by molecules of a gas located in the envelope (11) when an appropriate electrical discharge is produced between the anode (15) and the cathode (16) , characterized in that a heat exchanger (18) is also arranged inside said envelope (11).  12. Device according to claim 11, characterized in that a temperature probe (26 ') is also arranged inside said envelope, of the heat-transfer fluid circulating in said exchanger (18) and in an external regulation unit ( 9) connected to said temperature probe (26 ′) in order to regulate the temperature internal to said envelope (11) within a predetermined range.  13. Device according to any one of claims 1 1 or 12, characterized in that said gas is CO2 and in that said exchanger is adapted to regulate said internal temperature at 27 + 2 "C.  14. Device according to any one of claims il to 13, characterized in that said laser source comprises a fan (14) also disposed inside said envelope (11) for circulating therein the gaseous medium which it contains.  15. Device according to claim 14, characterized in that said fan extends over the same length as the anode (15) and the cathode (16) and in that it comprises a casing (21) having an opening d intake (22), a discharge opening (23) facing the space between the anode (15) and the cathode (16) as well as a rotor (24) for circulating the gaseous medium in the casing , in the direction going from said intake opening (22) to said discharge opening (23), in order to circulate the gaseous medium contained in the envelope (11) so that there is a flow leaving the 'discharge opening (23) and passing through the space between the anode (15) and the cathode (16) before heading towards the intake opening (22).  16. Device according to claim 15, characterized in that it further comprises a heat exchanger (18) disposed inside said envelope so that said flow of gaseous medium passes through the exchanger (18) between the anodes and cathode (15, 16) on the one hand and said intake opening (22) on the other hand.  17. Device according to any one of claims 14 to 16, characterized in that in the path between the anode and cathode (15, 16) on the one hand and said intake opening (22) on the other hand, is find an outlet (13) and an inlet (12) of gaseous medium in said envelope (11).  18. Device according to any one of claims 11 to 17, characterized in that it comprises a waveguide formed by a flexible tubular body (6) covered internally with a reflective dielectric coating, with the hollow of said body which is filled with a gas very inert to ionization.  19. Device according to claim 18, characterized in that said gas fills said hollow at a pressure slightly higher than atmospheric pressure.  20. Device according to any one of claims 18 or 19, characterized in that said gas is helium.  21. Device according to any one of claims 1 1 to 20, characterized in that it is provided with a head for applying a laser beam comprising optical processing means (34) so that said laser beam illuminates on the support to be cleaned (3) a tapered area (32), that is to say extending essentially in width while its thickness is small.  22. Device according to claim 21, characterized in that said optical processing means comprise a cylindrical lens (34) whose position corresponds to the working distance provided between said head (4) and the support (3) to be cleaned.  23. Device according to any one of claims 11 to 22, characterized in that it is provided with a head for applying a laser beam comprising means for blowing or sucking the vapors and dust resulting from the vaporization by the laser beam of the material to be removed.  24. Device according to claim 23, characterized in that said head comprises means of suction of said vapors and dusts, provided with a filtration element for retaining said dusts.  25. Device according to any one of claims 11 to 24, characterized in that it is provided with a head for applying a laser beam comprising means (46) for detecting the presence of a facing metal. of said head (4), in order to prohibit the emission of a laser beam in the absence of metal.  26. Device according to any one of claims II to 25, characterized in that it is provided with a head for applying a laser beam carrying contact means (43) with the support (3) to be cleaned in order fix the working distance between the head (4) and the support (3).  27. Device according to claim 26, characterized in that said contact means comprise at least one roller (43) intended to be driven on said support.