Graffiti is a near constant problem for historic structures and monuments, particularly in urban environments. Currently available non-destructive techniques are not always adequate for graffiti removal. Acrylic spray paints, the most common graffiti material, are composed of polymers which are subject to degradation by ultraviolet light. The feasibility of using this mechanism as a tool for graffiti removal was explored through practical experiments. Red acrylic paint was sprayed onto samples Indiana limestone which were then exposed to UV-B light sources for two, four, and six weeks. Following exposure, samples were scrubbed with acetone, a common solvent used in graffiti removal. Relative differences in the paint removal on the treated and untreated sides were observed, with more paint being removed from the treated sides in some cases. Additional samples were coated with titanium dioxide and exposed to ultraviolet light, yielded a greater level of overall paint removal. It was concluded that this mechanism offers promise and warrants further study. / Department of Architecture

Thesis (MSc)--Stellenbosch University, 2011. / ENGLISH ABSTRACT: Lasers are commonly used in the industry for various applications such as laser cutting, laser drilling, lithography, medical applications, surface cleaning and a myriad of other applications. In any application of a laser the beam properties are significant. In the paint removal application discussed in this thesis, the beam properties of the laser beam can have a large impact on the efficiency of the paint removal process. The pulse energy or the average output power of the laser is normally an important parameter in laser materials processing applications. The spatial profile or intensity distribution of the beam also has an influence on the process. The propagation of the laser beam from the laser to the working point is also significant in applying the laser beam to the material. In the ideal scenario one would like to control all the parameters of the laser in terms of the output, in energy or output power, the propagation of the laser beam and the intensity distribution of the beam. The process of laser-based paint removal is no different to this. In this process a TEA CO2 laser is used for the ablation of paint from a substrate. In this application high pulse energy is required from the laser together with good beam propagation properties for delivery of the beam over a long distance. For this application the multimode beam of the TEA CO2 laser can be applied for the paint removal. The multimode beam has sufficiently high pulse energy for the paint removal process, but is not suitable for propagating over long distances through a beam path with a finite aperture. Furthermore the multimode beam does not have a uniform energy intensity distribution. It would therefore be ideal if the TEA CO2 laser could be designed with a custom beam that has a uniform intensity distribution, high pulse energy and good beam propagation. These requirements lead to the study of flattened irradiance profile laser beams. In this thesis flattened irradiance profile beams in the form of Flattened Gaussian beams are investigated. The theory of the Flattened Gaussian profile as well as the propagation of the beam is investigated. Furthermore the generation of such a beam internally to the laser resonator is studied. In succession to this a custom laser resonator was designed and implemented on the TEA CO2 laser. The resulting Flattened Gaussian Beam was characterised and applied to the application of laser paint removal. It was finally shown that the Flattened Gaussian Beam could be successfully generated and applied with equal success in the application of laser paint removal. / AFRIKAANSE OPSOMMING: Lasers word algemeen in die industrie gebruik vir toepassings soos laser snywerk, laser boorwerk, litografie, mediese toepassings, oppervlakreiniging en verskeie ander. In enige toepassing van 'n laser is die eienskappe van die laserbundel van groot belang vir die proses. In die verf verwydering toepassing wat bespreek word in hierdie tesis het die bundel eienskappe 'n groot invloed op die effektiwiteit van die verf stropings proses. Die pulsenergie of uitset drywing van die laser is gewoonlik 'n belangrike parameter in 'n materiaalverwerkings toepassing. Die ruimtelike profiel of energie intensiteitprofiel van die bundel het ook 'n invloed op die proses. Die voortplanting van die bundel vanaf die laser na die werkspunt het ook 'n beduidende invloed op die toepassing van die laserbundel op die materiaal. In die ideale geval sal mens graag al die parameters van die laserbundel soos pulsenergie of drywing, die bundel voortplanting en energie intensiteitprofiel wil beheer. Die toepassing van die laser vir verfverwydering vereis ook die beheer van hierdie unieke parameters wat reeds genoem is. In hierdie proses is 'n TEA CO2 laser gebruik vir die verwydering van verf van 'n substraat. Die toepassing vereis hoë pulsenergie saam met goeie bundel voortplantingseienskappe vir lewering van die bundel oor lang afstande. Die multimode bundel van die laser kan gebruik word vir hierdie toepassing. Die multimode bundel bevat genoegsame energie vir die verfstropings proses maar is nie geskik vir voortplanting oor lang afstande deur 'n bundelpad wat 'n beperking op die bundel grootte het nie. Verder het die multimode bundel ook nie 'n uniforme energie intensiteitprofiel nie. Dit sou ideal wees as die TEA CO2 laser toegerus kon word met 'n toepassingsgerigte bundel wat hoë puls energie, goeie bundel voortplanting en 'n uniforme intensiteitprofiel het. Hierdie vereiste het gelei tot die studie van laserbundels met 'n uniforme plat energie intensiteitprofiel. In hierdie tesis word plat intensiteit bundels in die vorm van plat Gaussiese bundels ondersoek. Die teorie van plat Gaussiese bundels sowel as die voortplanting van hierdie bundels word hier ondersoek. Verder word die opwekking van hierdie bundels intern tot die laserresonator ook ondersoek. Na die ondersoek is daar oorgegaan in die ontwerp en implementering van 'n doelgemaakte resonator op 'n TEA CO2 laser. Die resonator het 'n plat Gaussiese bundel as uitset gelewer. Die bundel was gevolglik gekarakteriseer en aangewend in 'n verfstropings toepassing. Ten einde is daar getoon dat 'n plat Gaussiese bundel suksesvol opgewek en toegepas kon word.