The present invention relates to a method of operating a laser for the removal of a coating from a substrate.

Laser coating removal forms an interesting application with a multitude of industrial clients and potential environmental, cost and quality advantages over existing processes. However, in order for the industry to commercially accept the process, a removal rate of greater than 10 mil-ft ² /kW-min needs to be demonstrated for most coatings. Currently ns-pulsed near- infrared lasers up to 1 kW with 100 mJ pulse energies and CW near-infrared up to 10 kW demonstrated the rates up to 3 mil-ft ² /kW-min for the paints. CO2 lasers with up to 20 kW CW and Quasi-CW demonstrated the removal rates less than 10 mil-ft ² /kW-min for typical paints and coatings. Alternatively, low energy pulses of 0.1 to 12 mJ may be used at high pulse repetition rates of 100 to 1000 kHz, achieving higher coverage range per raster while being focused into adequately small spots to maintain irradiance levels above the ablation threshold, resulting much lower removal rate smaller than 10 mil-ft ² /kW-min, as the increase in coverage rate by decrease of spot size and pulse energy mathematically results in slight increase of removal rate, in reverse proportion to pulse energy. However, the increase in removal rate is limited by the size of the smallest spot size practically and theoretically attainable. It is thus necessary to develop a methodology for coating removal with higher efficiency in removal speed.

Two methods for the removal of surface coatings using lasers are known. The first method is known as the ablation method and comprises ablation of the surface using high frequency low energy laser pulses, where the coating is gradually removed stepwise from the top or outermost down. This method is best suited to opaque coatings that strongly absorb the laser radiation. The second method is best suited to highly transparent coatings on opaque substrates, wherein a substantial portion of a high energy laser pulse passes through the surface coating and is absorbed at the interface between the coating and the substrate. This results in the ablation (i.e. the vaporisation) of material at the interface, generating large pressures at the interface causing detachment and ejection of the coating. This method may be referred to as the detachment method. Neither of these methods are well suited to semi- transparent coatings. Accordingly, it is desirable to develop methods for the removal of such coatings.

Pulsed laser coating removal by detachment and ejection, Roberts, D. E., Appl. Phys. A 79, 1067-1070 (2004) discloses the pulsed laser removal of paint coatings from substrates by detachment and ejection of the entire layer. The dependence of the removal efficiency on fluence, coating thickness and pulse duration were examined. For weakly absorbing

1

SUBSTITUTE SHEET RULE 26 coatings on easily ablating substrates, efficiencies as high as 800 μηι cm ² J ^-1 have been measured, i.e., more than two orders of magnitude higher than obtainable by surface ablation.

The invention is set out in the claims. Given a finite laser power budget and semi-transparent coating, the disclosed method provides an increase in efficiency over an ablation-only process or a detachment-only process. Using a 200mJ 1 μηι YAG laser, for a white paint with primer, a greater than 20 mil-ft ² /kW-min removal rate can be achieved. This process rate can be improved using higher energy and a larger beam. This is achieved by providing a laser pulse energy such that a portion of the semi-transparent coating is removed by ablation and the remaining portion is detached from the surface.

Embodiments of the invention will now be described by way of example with reference to the drawings of which:

Figure 1 is a graph of laser beam intensity versus material depth;

Figure 2 shows the ablation of a coating adhered to a substrate;

Figure 3 shows the detachment and ejection of a coating adhered to a substrate;

Figure 4 shows the alternative removal method of the present invention;

Figure 5 is a graph of removal rate versus pulse energy for white paint with a primer undercoat;

Figure 6 is a graph of removal rate versus pulse energy for a range of surface coatings;

Figure 7 is a graph of coating removal rate as a function of pulse energy and pulse rate;

Figure 8 is a graph of coating removal efficiency as a function of pulse energy for the ablation and detachment methods;

Figure 9 shows an example of a laser apparatus for removing a coating from a substrate; Figure 10 shows an example of beam delivery optics;

Figure 11 shows an illustration of a laser beam being absorbed by a metallic substrate where the thermal diffusion length is greater than the optical absorption length;

Figure 12 shows an example of focussing optics arranged to deliver a laser beam onto separate areas of a substrate; Figure 13 shows an example of how the threshold parameters for detachment vary with film thickness, pulse energy and coating absorption coefficient.

When a laser beam reaches a surface coating, the intensity of the beam decays in accordance with the Beer-Lambert law, as shown in Figure 1 , where l _t is the beam intensity at depth t and lo is the intensity of the beam when it reaches the surface coating.

For a strongly absorbing or opaque surface coating, the beam only penetrates a short distance into the surface before being absorbed. This results in rapid heating localised at the portion of the coating first reached by the beam. Accordingly, this portion is vaporised or ablated. This is illustrated in Figure 2, where a thermally affected portion 107 of the coating 101 (adhered to substrate 103) is ablated by a laser beam 105. This is referred to as the ablation method, wherein the entire coating is ablated stepwise using multiple laser pulses from the outermost of the coating. Using the ablation method, coating removal efficiency is maximised when the beam energy (and therefore beam fluence for a given beam radius) is just above the ablation threshold (i.e. the minimum energy for the surface to be ablated) and the maximum pulse frequency achievable at that beam energy, given a finite laser power budget.

As stated above, the existing known methods are the ablation method that comprises ablation of the surface using high frequency low energy laser pulses, where the coating is gradually removed stepwise from the top or outermost down, and the detachment method, best suited to highly transparent coatings on opaque substrates, wherein a substantial portion of a high energy laser pulse passes through the surface coating and is absorbed at the interface between the coating and the substrate, resulting in the ablation (i.e. the vaporisation) of material at the interface causing detachment and ejection of the coating.

For a weakly absorbing or transparent surface coating, the beam penetrates the surface coating and reaches the substrate, where it may be absorbed if the substrate or interface is strongly absorbing. The beam intensity reaching the substrate is high as the coating is weakly absorbing. As depicted in Figure 1 1 , in the case of a substrate which strongly absorbs the laser wavelength and is quite thermally conductive, such as a metal, such a substrate has a short optical absorption length 1101 , corresponding to 1/a (as defined below). Such a substrate also has a thermal diffusion length 1102 during the laser pulse irradiation time τ, corresponding to V(DT) which is typically longer than the optical absorption length 1101. As shown in Figure 3, laser beam 105 passes though coating 101 and is absorbed at substrate 103. The thermally affected portion 107 is ablated resulting in the build-up of pressure between the coating 101 and the substrate 103. If the pressure is large enough, the substrate 103 above the thermally affected portion 107 is detached and ejected. This is referred to as the detachment method, wherein the entire coating is detached and ejected from the surface with one laser pulse. Typically, the interface features an onset irradiance threshold for decomposition and onset fluence for generating adequate interfacial pressure, sufficient to counteract shear forces at the edge of the irradiated area of the coating. The latter is commonly referred to as detachment fluence onset or threshold.

Therefore, when using the detachment method, it is desirable to maximise the pulse energy to ensure that the onset irradiance threshold is met at the interface between the coating and surface. Therefore, maximum coating removal efficiency is achieved when the pulse energy is high (allowing the laser beam diameter to be increased). Typically, coating removal efficiencies using the detachment method are much higher than using the ablation method.

The present invention employs an alternative method. As illustrated in Figure 4, pulses of a higher energy than would be typically used in the ablation method are used to ablate one or more portions 1 11 of the coating 101. After one or more pulses when the thickness of the coating has been reduced, owing to a combination of the reduction in coating thickness, semi-transparency of the coating and high beam energy, the beam intensity at the interface between the coating and the substrate meets the onset irradiance threshold for

decomposition and onset fluence for generating adequate interfacial pressure, which is the intensity at l _t h, detaching and ejecting the coating (as illustrated in Figure 3).

The alternative method typically concerns, but is not limited to, laser coating removal conditions where the coating or top layer is highly or semi-transparent to the laser wavelength, while the laser radiation can be absorbed by the substrate or interface of the coating and substrate, or cause severe desorption, outgassing or rapid decomposition at the interface. The absorption coefficient of a semi-transparent material is 250-5,000,000 rrr ¹ . The absorption coefficient of a semi-transparent material may be at least 2,500 rrr ¹ , preferably at least 25,000 rrr ¹ . The absorption coefficient of a semi-transparent material may be less than 500,000 rrr ¹ , preferably less than 100,000 rrr ¹ . In order to increase the rate of coating volume removal, the alternative method may employ distribution of the average rate of laser energy release (pulse frequency) in fewer pulses per unit time of higher pulse energy and/or intensity, rather than many pulses of low energy (ablation method) or one pulse of high energy (detachment method). Accordingly, the pulse energy of the alternative method is between 1 to 5 J per pulse. The pulse energy may be greater than 10 μ , preferably greater than 100 μ . The pulse energy may be less than 1 J, preferably less than 0.1 J. The pulse fluence may be less than 100 J cm ^-2 , preferably less than 10 J cm ^-2 . The pulse fluence may be greater than 0.01 J cm ^-2 , preferably greater than 0.1 J cm ^-2 . The pulse duration may be less than 10 ms, preferably less than 1 μβ. The pulse duration may be greater than 1 fs, preferably greater than 1 ns. The pulse frequency may be may be less than 5 MHz, preferably less than 300 kHz. The pulse frequency may be greater than 0.1 kHz, preferably greater than 1 kHz. The invention considers maintaining average laser power and pulse duration at the same values. The higher energy pulses thus achieve two effects: 1) The higher intensity helps to reach the threshold irradiance for decomposition through a thicker semi-transparent coating in accordance with the Beer-Lambert law. 2) The higher fluence distributed over a larger area achieves the detachment onset more easily as it raises interfacial pressure and thus detachment force over a larger area. Higher energy pulses achieve decomposition threshold through thicker coatings and lower detachment threshold.

The alternative method is particularly effective for removing semi-transparent coatings that are too thick or absorbing to be removed by the detachment method but also not strongly absorbing such that ablation is inefficient.

The inventors have discovered that when ablating a semi-transparent material, a non-linear change in removal rate is achieved above a pulse energy threshold, where average power and pulse duration are fixed. This is illustrated in Figure 5, which shows removal rate of a coating of white paint with a primer on aluminium, which is semi-transparent at the wavelength of the laser radiation used (1064 nm). As the pulse energy is increased at a given frequency, the removal rate is expected to increase continuously and approximately linearly, as seen between a pulse energy of 1 mJ and 101 mJ in Figure 5. However, at around a 110 mJ pulse energy, there is a nonlinear increase in removal rate. It has been established that this increase in removal rate arises from meeting the onset irradiance threshold for decomposition of material at the coating to substrate interface and generation of adequate interfacial pressure being reached due to this decomposition, causing detachment of the coating.

When the ablation method is used, the highest removal rate efficiency (i.e. rate of coating removal per unit of laser power) is achieved at the highest pulse frequency attainable at a pulse energy just above the threshold energy for ablation. However, Figure 5 illustrates that it is feasible efficiency can increase rather decrease when pulse energy is increased. This phenomenon has also been confirmed for other coatings, as shown in Figure 6 which shows removal rate as a function of pulse energy with a pulse frequency of 10 kHz.

It can also be shown that higher energy pulses at the expense of lower pulse frequency can improve removal efficiency. The following equations relate to the method shown in Figure 4. The coating 101 and substrate 103 are irradiated by a laser beam 105 with radiation at a set wavelength defined by the lasing medium of choice and by tuning of the laser oscillator. The laser radiation is delivered in pulses with a duration ranging from 1 attosecond to several minutes, sequenced in bursts consisting of one or more than one pulses, adequate to irradiate a designated area of the material as the beam changes position in relevance to the material surface and vice versa. The laser irradiation, coating 101 and substrate 103 parameters are defined as follows: β is the thickness of the coating; a is the absorption coefficient equal to 4ττκ/λ where κ is the complex refractive index (where the optical absorption length is the reciprocal of the absorption coefficient); λ is the laser wavelength; p is the density; c is the heat capacity; τ is the laser pulse duration; D is the thermal diffusivity; k is the thermal conductivity; lo is the light intensity reaching the material; Ds is the thermal diffusivity of the substrate; Dc the is thermal diffusivity of the coating; Tas the is vaporisation temperature of the substrate; Tac is the vaporisation temperature of the coating; Res is the reflectivity of the coating-substrate interface; Rc is the reflectivity of the coating-air interface; F is the pulse fluence; PWa is the average laser power and PW _Pk is the peak laser power of each pulse.

With the assumption that a coating is optically much thicker than the thermally affected zone (TAZ) during one laser pulse (Equation 1) and the substrate TAZ is much thicker than the optical penetration (Equation 2), such as in the case of polymer or organic based coatings applied on metallic substrates irradiated with 1 μηι radiation, a comparison is conducted on whether the irradiation can ablate the surface of the coating directly irradiated by the laser pulse or not.

- » y[Dr (Equation 1) a (Equation 2)

If the conditions satisfy Equation 3, then for a typical case such as for paints and most common coatings, the ratio of absorption coefficient to the product of material density and heat capacity is large enough (Equation 4) that the temperature at the top of the coating can be raised above the ablation threshold (Equation 5).

(Equation 3) a

pc (Equation 4)

(1 — R _c )a ^■ F ^■ e ^~ax

so that — > T _ac (Equation 5) Alternative method coating removal can take place in a manner such that the partially transmitting coating is ablated with adequate pulses (i.e. reduced in thickness) until enough irradiance can be transmitted through the remaining coating thickness to induce the detachment method and detach it from the substrate. An adequate pulse can also be a proportion of a single pulse, whereas the first part of the pulse ablated the surface of the coating and reduces its thickness such that the pulse irradiance reaching the coating- substrate interface is capable of detaching the coating.

Assuming that the process is performed with a fixed average laser power budget PWa distributed in pulses of equal energy and fixed pulse duration, the pulse energy can be adjusted to an optimum defined by the decline of ablation efficiency with the increase of pulse energy and the increase in detachment efficiency with the increase of pulse energy. In the alternative detachment process, the efficiency is a proportional sum of the partial ablation and detachment efficiencies (Figure 8).

Figure 7 shows the predicted removal rate (i.e. ablation rate + detachment rate) as a function of pulse energy and pulse frequency, assuming a maximum laser power, for a semi- transparent material. Ablation rate is proportional to the pulse energy and frequency while detachment rate only increases with the pulse energy. Figure 7 shows how removal rate is expected to change as a function of pulse energy and pulse frequency for a typical combination of materials comprising a semi-transparent polymer coating on a metal substrate for 1 μηι laser radiation. It can be seen that, under the constraint that the total laser power budget is fixed, the higher removal rate is not achieved at the lowest or highest pulse energies.

Figure 13 shows an example of how the threshold radius of the detached spot/laser beam radius varies with pulse energy and coating thickness D. The values are for a substrate reflection (Rs) of 0.1 and threshold energy density for detachment (Fth) of 0.2 J cm ^-2 . The absorption coefficient a is 10, 20 and 50 mm ^-1 for Figure 13 a), b) and c), respectively. From a) it can be seen that even using a low energy pulse of 0.1 J, 500 μηι diameter detachment is possible for a 0.37 mm thick coating. From b), which is for a more strongly absorbing coating, it can be seen that 500 μηι diameter detachment is only possible for a 0.19 mm thick coating at this energy. From c), which is for a yet more strongly absorbing coating, it can be seen that 500 μηι diameter detachment is not possible for this energy; however, 100 μηι diameter detachment is possible using a 0.25 J pulse.

In summary, the inventors have discovered that coating removal efficiency for semi- transparent coatings can be increased by increasing pulse energy while lowering pulse frequency, maintaining the same power. This is contrary to standard practice wherein pulse frequency is maximised if the ablation method is used or pulse energy is maximised if the detachment method is used.

As an example, the laser source is a diode pumped Nd:YAG laser with q-switching to control the pulsing of the laser. Q-switching is not essential and pulsing may be controlled by other means, e.g. via electrically controlling seed pulses injected into the laser resonator. Pumping can also be performed with methods other than a laser diode, e.g. by flash lamp pumping. The amplifying medium of the laser may be composed of a selection of other crystals or glasses. The source may also be fibre based or a mixture of fibre and crystal block amplifiers and oscillators, q-switched or not. The source may also be of semiconductor nature, such as laser diodes. The source may also use gas or liquid media of amplification, such as dye, CO2, N2, combinations of noble gasses with halogens and other combinations of gasses. The source may also be mode-locked, or amplified in a supercontinuum medium, further amplified and tuned via optical parametric amplification. The laser source may emit a second, third or other frequency multiplication harmonic, filtered or in combination with all other emitted harmonics. A combination of the above lasers and wavelengths may also be used for emitting the necessary radiation.

1.0 μηι pulsed lasers (solid state YAG or fibre lasers) are semi-transparent for most paints and are preferred. 0.5 μηι, 1.5 μηι or 2.0 μηι laser radiation may also be used, depending on the coatings and paints. The wavelength of the radiation may be may be between 0.2-2.0 μηι, preferably 1.0-1.5 μι ι.

The beam or beams are typically transmitted from the source to the coating and substrate by a fibre optic cable. The beam or beams can also be transmitted via an open beam setup using deflective and refractive optics.

As illustrated in Figure 12, the laser beam 105 emitted by the laser processing head 913 may be moved in relation to the substrate 103 with the use of deflective or refractive optics 1201 , for example mirrors or prisms respectively. These optics may be electromechanically moved, rotated and controlled. The beam motion may also be controlled by an acousto-optic device or other electro-optic device. The coated material can also be moved in reference to the beam, to give the same results, or both beam and material can be moved together, simultaneously in independent directions. The whole assembly comprising the scanners, fibre exit, other electronics, water, air feeds and gas extraction, and laser source box can also be moved in relation to the coated surface.

The beam may be moved across the surface of the coating/substrate at a speed that may be defined by other process considerations e.g. for a 10 kHz pulse repetition rate for a 100 μηι thick acrylic based white paint, where the beam diameter at the surface of the paint is 3 mm, may be moved at 20m/s relative to the coating/substrate, or at 3 m/s for a 500 μηι thick coating. The beam can follow linear, raster, circular or other patterns defined by vector components or curved trajectories. Release of pulses can be continuous over a beam trajectory being transcended or sporadic in bursts.

The beam is focussed by a single lens or a lens system of f-theta type or f-theta telecentric, or other type of focussing lens. The beam may also be focussed by a lens system that forms a line focus, an oval focus or a rectangle focus. Sensors 915, such as those shown in Figure 9, may be included with or next to the scanner or beam delivery optics to sense the emissions of the ablation plasma and fire, the distance from the coated material being processed, the reflection of the laser or other light from the coated material surface. The sensors may also sense other properties like dielectric permeability, refractive index, scattering, reflection or scattering of other electromagnetic regions like t-rays, acoustic reflections and scattering, etc. A camera may also be included next to or with the scanning optics or focussing optics. The beam delivery optics may be focusing the beam on the coated material being processed or may direct a collimated or divergent beam on the coated material, as illustrated in Figure 10 discussed below.

The equipment may also contain extraction inlets, ducts and filtration systems to collect and manage the process waste produced by the removal process, such as the vacuum extractor and filtration system 903 shown in Figure 9.

The beam may be moved across the surface of the coated material with a speed and a pulse repetition rate that allow each pulse to impinge on the material overlapping 100% or to a smaller percentage or not to overlap at all. Each pulse may be ablating material from the coating or detaching material from an area of the material that has received more pulses before the current one.

Figure 9 shows an example of a laser apparatus for removing a coating 101 from a substrate 103. It shows a laser 901 that generates a laser beam 105 that is transmitted via a beam delivery fibre 905 to beam delivery optics 911 in a processing head 913. The removal products are removed using a vacuum extractor and filtration system 903. Optics signal interface 907; sensor signal interface 917; sensors 915 to sense the emissions of the ablation plasma and fire; and closed loop control system 909, that uses local closed loop control logic and collects signals from the sensors 915 and may change the speed and orientation of mirrors via the sensor signal interface 917 and the focus of optics via the optics signal interface 907 within the beam delivery optics 911 , are also located in the processing head 913. As an example, the apparatus of Figure 9 may have a scanning field up to 10 cm, a max speed of 25 cm/s, up to 2.4 kW average power, 500 μηι minimum spot size, 152 J/cm ² fluence and 3 GW/cm ² irradiance.

Figure 10 shows an example of beam delivery optics including mirrors 1001 to direct the laser beam 105 to the coating 101 and substrate 103.

The coating 101 comprises one or more layers comprising materials that are semi- transparent to the wavelength of the laser. Layers closer to the substrate may be fully transparent to the laser wavelength. Anti-reflection coatings may also be included in the sequence of coating layers. The coating materials may be one of or a combination of the following, polymer based, gelatines, glass, crystal, polycrystalline material like alumina, zirconia, aluminium nitride, titania, silicon carbide, silicon nitrate, tungsten carbide, organic material, organic crystal, diamond, salt, salt hydrate. The coating layers can be continuous or intermittent, fibrous or porous, or forming a metamaterial. The coating layers may be in pure form or have impurities, dopants, additives or discontinuities. The additives and other discontinuities may contribute to the partial absorption of the coating layers. Distribution of additives and discontinuities may be homogeneous or inhomogeneous, or following a predefined gradient.

The substrate 103 can consist of one or more layers where the layer closer to the coating 101 is adequately absorbing at the laser wavelength. If the substrate is layered, a layer closer to the coating may also reflect the laser wavelength depending its refractive index and the refractive index of the coating, pursuant to Snell's law of refraction. A substrate layer or layers closer to the coating should limit the transmission of the laser radiation inside the substrate materials, minimising transmission further than, for example, 5 μηι. The substrate layer closest to the coating may be infused with additives that will increase absorption of the laser radiation, or desorption rate of the material once light has been absorbed. The substrate surface may have been roughened prior to applying the coating layers for enhancement of coating adhesion as well as enhancement of interaction with the laser light during detachment. The substrate 103 may be metal, such as aluminium or its alloys, titanium, iron or iron based steel, nickel or nickel based steel, cobalt or cobalt based steel, copper, brass, tungsten, a platinum alloy, gold, silver, zinc, tantalum, tin, zirconium, or an alloy of the above or a mixture of the above. The substrate 103 may be a semiconductor like silicon, GaAs, aluminium nitride, CdTe, germanium, gallium nitride and others. The substrate 103 may be a ceramic or crystal or polycrystalline material that adequately absorbs the laser light. Additives or doping may be included in the ceramic or crystal to help the material absorb the laser light or to decompose during irradiation with the laser light. The substrate 103 may be a composite such as carbon fibre reinforced polymer, glass reinforced polymer, ceramic composite, wood or other, where either the binder or the fibre or powder reinforcements can absorb the laser light.

The material of the substrate 103 may also contain an added layer or mono-atomic layer at the interface of the coating and the substrate to absorb the laser radiation, or to rapidly decompose under the laser radiation, or to enhance decomposition of the substrate or the coating during the laser irradiation. This interface additive may also consist of sporadic particles, discontinuities, roughness or fibres.

The coating 101 may have a relatively low absorption coefficient so that the reciprocal of this coefficient, indicating length, is smaller than the thermal diffusion length, i.e. satisfying the inequality of Equation 1. The inequality is satisfied for the case of most polymers which are not black or do not have a high content of light absorbing additives. A typical example is polymers like acrylic paint, polyurethane paint, polycarbonate coatings, poly-Teflon® coatings, PVC paint or coatings, where the material is mostly clear or significantly transparent to near infrared wavelengths such as 1064nm, 808 nm or 940nm, while being good thermal insulators, therefore, the optical transmission length is 20 to 100 times longer than the thermal diffusion length. Other coatings that may satisfy this initial requirement are glass, crystal and ceramics which are good insulators and can be adequately transparent. A special case that still satisfies the inequality for a wavelength bandwidth between 200 nm and 15 μηι is clear or semi-transparent diamond, which is highly thermally conductive and yet has an optical absorption low enough to transmit adequate light to the substrate 103.

The substrate 103 is typically a metal, for example aluminium, for which the free electron cloud due to the metal bonds strongly absorbs any light penetrating the surface, as well as conducting heat very efficiently. Most metals and specifically highly electrically conductive metals like copper, silver, gold and aluminium satisfy the inequality of Equation 2. For conductive metals the optical transmission length is hundreds of times less than the thermal diffusion length. However, most metal substrates will satisfy the inequality, such as iron, steel, titanium and others as listed above. Other examples such as graphite, semiconductors or polymers with light absorbing additives may also satisfy the inequality with a ratio of at least 1 to 30.

If a layer, film, additive or treatment has been applied intentionally at the coating to substrate interface to reduce reflectivity at the interface, or increase optical absorption at the laser wavelength used, then Equation 3 should be considered for the applied layer or resulting surface of the substrate. After 1 or several steps of thickness reduction via surface ablation of the coating, the coating will eventually reach a thickness where enough laser radiation can be transmitted to satisfy Equation 1 to ablate the substrate or additives at the interface as per Equation 3 and thus cause detachment. The condition is well satisfied by polymer coatings and paints, e.g. white paints, which comprise a number of additives and discontinuities that absorb enough light, while their density and thermal conductivity are low enough to satisfy Equation 4 where large is relative to the coating ablation temperature and in the typical case above 0.1. The condition can be satisfied by, for example, coloured paints and polymer coatings, coloured glasses, semi-opaque ceramics and semi-transparent semiconductors.

The method of the present invention is suitable for removing a coating from a substrate using pulsed laser radiation to reduce the coating thickness by ablation until the coating is detached from the substrate by optically induced detachment. Optically induced detachment comprises the following: a. Detachment is induced by the laser radiation being absorbed on the substrate at close proximity to the coating and heating up a layer of the substrate to evaporation.

b. Detachment is induced by the laser radiation being absorbed on the substrate causing physical desorption of a layer of the substrate.

c. Detachment is induced by the laser radiation being absorbed by a layer

closest to the substrate and heating this layer to evaporation. d. Detachment is induced by the laser radiation being absorbed on a coating layer closest to the substrate and causing physical desorption of this coating layer.

e. Detachment is induced by the laser radiation photo-dissociating a part of the substrate at close proximity to the coating.

f. Detachment is induced by the laser radiation photo-dissociating a part of the coating at close proximity to the substrate.

g. Detachment is induced by the laser radiation destroying Van Der Waals or hydrogen bonds at the coating to substrate interface, thus reducing adhesion of the coating to the substrate.

h. Detachment is induced by the laser radiation being absorbed by a sacrificial layer at the interface between the coating and the substrate, causing the sacrificial layer to vaporise.

i. Detachment is induced by the laser radiation being absorbed by a sacrificial layer at the interface between the coating and the substrate, causing physical desorption of the sacrificial layer. j. Detachment is induced by the laser radiation photodissociating a sacrificial layer at the interface between the coating and the substrate.

k. Detachment may be induced by the laser radiation destroying Van der Waals or hydrogen bonds between a sacrificial layer at the coating to substrate interface, and the coating or the substrate or both.