ИСТИНА

There has been considerable recent interest in the application of laser radiation in restoration of objects of cultural heritage [1, 2]. In particular, laser radiation is used for cleaning of paper materials. For example, nanosecond pulsed laser radiation at a wavelength of 532 nm cleans paper surface from dirt or ink better than an eraser does without damaging the base layer [3]. Laser cleaning involves ablation, since the absorption of radiation by particles of dust or ink is significantly greater than that of cellulose fibers. The results of [4, 5] show that nanosecond pulsed laser radiation at a wavelength of 532 nm can be used not only to eliminate surface contaminants, but also bleach paper (i.e., restore original optical properties or decrease discoloration). Such effects also result from laser ablation. In this work, we demonstrate that the paper ablation is a two-threshold process in which the first and second thresholds correspond to ablation of microparticles contained in paper material and cellulose fibers, respectively. We also demonstrate bleaching of historical paper under cw irradiation at a wavelength of 532 nm, a laser power of 1 W, and an irradiation spot of 2 mm. The degree of bleaching (decrease in discoloration) is quantitatively determined using the L*a*b* color coordinates. It is shown that a variation in the L* coordinate depends on the exposure time. A significant distinctive feature of paper bleaching under cw irradiation is almost complete absence of degradation of ink on the surface of paper. Prospects for application of laser ablation in the study and restoration of historical papers are discussed.