PROCESS AND APPARATUS FOR MONITORING SURFACE LASER CLEANING

Abstract: A process and apparatus for monitoring the cleanign of a surface (1) by laser irradiation include illuminating the surface (1) to be cleaned, being cleaned or having been cleaned by ultra-violet irradiation provided either by the cleaning laser or by a further source (5) sufficient to excite into fluorescence contaminants (10, 11) on the surface (1) which fluoresce under ultra-violet irradiation.

Description: PROCESS AND APPARATUS FOR MONITORING SURFACE LASER CLEANING This invention relates to a process and apparatus for monitoring the cleaning of a surface by laser irradiation. In many manufacturing processes such as for aircraft and automobiles components have to be cleaned prior to joining or coating. Conventionally this task is performed by wet cleaning using either water jets or solvents. In both cases the liquid residue remaining after cleaning has to be disposed of. Due to the impact of the solvents or the residue containing liquids upon the environment and the costs associated with the disposal of these chemicals, alternative cleaning techniques are desirable which do not involve the use and disposal of environmentally hazardous chemicals or residues. It has been proposed to use a cleaning process which involves the irradiation of a surface by laser to remove contaminants such as solvents or epoxy particles. However it is also advisable to monitor the effectiveness of such laser cleaning processes and techniques have been proposed for doing this by plasma or ion emission analysis. One of the disadvantages of such monitoring techniques is that they can only monitor the very area the laser pulse is cleaning and no analysis can be performed on the area that is to be cleaned or the area that has been cleaned. Additionally the analysis process is time consuming and relatively expensive to carry out. There is thus a need for a generally improved and simplified process of monitoring the cleaning of a surface by laser irradiation which can be used more effectively across the whole area of the surface to be cleaned. According to one aspect of the present invention there is provided a process for monitoring the cleaning of a surface by laser irradiation in which the surface to be cleaned, being cleaned, or having been cleaned is illuminated by ultra-violet irradiation sufficient to excite into fluorescence contaminants on the surface which fluoresce under ultra-violet irradiation. Preferably the intensity of fluorescence or the rate of decay of fluorescence is measured to indicate the area and thickness of contaminant on the surface. Conveniently the intensity or rate of decay measured is utilised controllably to vary operating parameters of the cleaning laser. Advantageously the ultra-violet irradiation is provided by an ultra-violet lamp. Alternatively the ultra-violet irradiation is provided by an ultra-violet laser. According to a further aspect of the present invention there is provided apparatus for cleaning a surface by laser irradiation, including a source of laser irradiation and means for irradiating the surface with ultra-violet radiation before, during or after cleaning, to monitor the area and extent of contaminant on the surface. Preferably the apparatus includes a sensor for detecting the intensity or rate of decay and area of fluorescence excited on the surface by the ultra-violet irradiation. Conveniently the apparatus includes means for controllably varying operating parameters of the cleaning laser in response to the detected intensity or rate of decay and area of fluorescence. Advantageously the source of laser irradiation is an ultra-violet cleaning laser which additionally provides the means for irradiating the surface with the ultra-violet monitoring radiation. Preferably the source of laser irradiation is a non ultra-violet or an ultra-violet cleaning laser and wherein the means for irradiating the surface with ultra-violet monitoring radiation is an ultra-violet lamp. For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which; Figure 1 is a perspective diagrammatic view of apparatus for cleaning and monitoring a surface according to a first embodiment of the present invention, Figure 2 is a diagrammatic view from above of a surface partially cleaned according to the apparatus of Figure 1, and Figure 3 is a perspective diagrammatic view of an apparatus for cleaning and monitoring the surface according to a second embodiment of the present invention. The apparatus and process according to the present invention for monitoring the cleaning of a surface by laser irradiation basically involves illuminating the surface 1, to be cleaned, being cleaned or having been cleaned by ultra-violet irradiation sufficient to excite into fluorescence contaminants on the surface 1 which fluoresce under ultra-violet irradiation. Many materials which form contaminants on a surface to be cleaned or cleaned fluorescence when irradiated with ultra-violet light. Examples of this are epoxy type contaminants. The intensity of fluorescence or the rate of decay of fluorescence is measured such as by means of an optical recording system such as a charge coupled device camera 2 to indicate the area and thickness of contaminant on the surface 1. In the embodiments of Figure 1 the laser being used to clean the surface 1 is an ultra-violet source so that in this embodiment the ultra-violet irradiation 3 provides both the cleaning laser radiation and the monitoring ultra-violet irradiation. To this end the scatter from the cleaning laser irradiation beam 3 is used as the radiation required to cause fluorescence. Alternatively in the embodiment of Figure 3, the cleaning laser providing a beam 4 of cleaning laser irradiation is a non ultra-violet source. In this case the required ultra-violet radiation can be supplied from any suitable source of ultra-violet irradiation such as one or more ultra-violet lamps 5. In the embodiments of Figures 1 and 2, the laser beam 3 or 4 is passed through an aperture plate 6 to provide a parallel beam of radiation and from thence through an imaging lens 7 onto the surface 1. The radiation is not focused as a spot on the surface 1 but rather as a line which is scanned across the surface 1 preferably by indexing the surface 1 with respect to the imaging lens 7. Thus in Figures 1, 2 and 3 of the accompanying drawings an area to be cleaned is shown at 8. The cleaned area is shown at 9. As shown in Figure 2 which is a plan view of the surface 1 from Figures 1 and 3, in the area already cleaned by laser irradiation there is not direct fluorescence except in regions 10 which represent residual uncleaned areas of contaminants which will fluoresce under the ultra-violet irradiation. In the area 8 to be cleaned there will be fluorescence from the whole surface when irradiated with ultra-violet irradiation with a region of greater thickness contaminant 11 showing up with a different intensity of fluorescence or rate of decay or fluorescence from the remaining uncleaned area 8. Thus in the embodiments of Figures 1 and 3 when laser cleaning takes place, removed contaminant material and fumes is extracted via an extraction pipe 12. The fluorescence monitoring can be effected in different ways. For example as the laser 3 or 4 is cleaning an area 9 of the surface 1, the fluorescence reveals areas that have been cleaned successfully, areas that the laser has not cleaned successfully such as the areas 10 and areas 8 and 11 that the laser is yet to clean. Using the optical recording system such as the camera 2 which acts as a sensor for detecting the intensity or rate of decay of the fluorescence, the areas of residual contaminants on the cleaned surface area 9 can be recorded, such as in Figure 2 and used to assess the areas that the laser needs to return and clean. Alternatively fluorescence can be used to estimate the thickness of the contaminant such as in the area 11, the head of the area already cleaned such as in the area 8 to be cleaned, and the laser parameters can be adjusted such as by varying the laser powers or scan speed, to account for the different contaminant thickness and position. Frequently areas of greater thickness of contaminant such as the area 11 on Figure 2 has different fluorescence characteristics. This may be a different intensity of fluorescence or a different fluorescence decay time. This information is used to estimate the thickness of the film of contaminant at area 11 so that the cleaning laser parameters may be adjusted prior to cleaning of the area 11 to be optimum for the thickness of the contaminant film in this area. For example if the contaminant area 11 is greater in thickness than the surrounding area 8 then the laser irradiation exposure time can be increased in the area 11 only rather than over the whole of the area 8. It is to be understood that the ultra-violet lamps 5 may be employed for monitoring in the absence of an ultra-violet source cleaning laser but can also be used in addition to an ultra-violet source cleaning laser to augment the ultra-violet radiation from the latter source.

Claims: CLAIMS 1. A process for monitoring the cleaning of a surface by laser irradiation in which the surface to be cleaned, being cleaned, or having been cleaned is illuminated by ultra-violet irradiation sufficient to excite into fluorescence contaminants on the surface which fluoresce under ultra-violet irradiation. 2. A process according to claim 1, in which the intensity of fluorescence or the rate of decay of fluorescence is measured to indicate the area and thickness of contaminant on the surface. 3. A process according to claim 2, in which the intensity or rate of decay measured is utilised controllably to vary operating parameters of the cleaning laser. 4. A process according to any one of claims 1 to 3, in which the ultra-violet irradiation is provided by an ultra-violet lamp. 5. A process according to any one of claims 1 to 3, in which the ultra-violet irradiation is provided by an ultra-violet laser. 6. A process for monitoring the cleaning of a surface by laser irradiation, substantially as hereinbefore described and as illustrated in Figure 1, Figure 2 or Figure 3 of the accompanying drawings. 7. Apparatus for cleaning a surface by laser irradiation, including a source of laser irradiation and means for irradiating the surface with ultra-violet radiation, before, during or after cleaning, to monitor the area and extent of contaminant on the surface. 8. Apparatus according to claim 7, including a sensor for detecting the intensity or rate of decay and area of fluorescence excited on the surface by the ultra-violet irradiation. 9. Apparatus according to claim 8, including means for control1ably varying operating parameters of the cleaning laser in response to the detected intensity or rate of decay and area of fluorescence. 10. Apparatus according to any one of claims 7 to 9, wherein the source of laser irradiation is an ultra-violet cleaning laser which additionally provides the means for irradiating the surface with the ultra-violet monitoring radiation. 11. Apparatus according to any one of claims 7 to 9, wherein the source of laser irradiation is a non ultra-violet or an ultra-violet cleaning laser and wherein the means for irradiating the surface with ultra-violet monitoring radiation is an ultra-violet lamp. 12. Apparatus for cleaning a surface by laser irradiation substantially as hereinbefore described and as illustrated in Figure 1 or Figure 3 as modified or not by Figure 2 of the accompanying drawings.

Inventors: SIDHU JAGJIT, SIDHU, JAGJIT
Applicants: BRITISH AEROSPACE, SIDHU JAGJIT, BRITISH AEROSPACE PUBLIC LIMITED COMPANY, SIDHU, JAGJIT
Application year: 1998
Main classification: G01N21
IPC codes: B08B7, G01N21