LETTERS TO THE EDITOR

1 LASER STAIN REMOVAL

1.1 TO THE EDITOR:

I am writing to comment on the paper “Laser stain removal of fungus-induced stains from paper,” by Hanna M. Szczepanowska and William R. Moomaw (JAIC 33(1994):25–32). While I thought that the general content of the paper was interesting, there were a few errors and omissions that I think should be brought to the attention of interested readers.

First and foremost, the mechanism of removal of fungal stains by laser irradiation needs to be more carefully considered. The authors state that “the light from the laser is absorbed by the stain to produce intense local heating within the stain, ink, or pigment and vaporize them from the paper fibers.” Indeed, the mechanism may be one of selective absorption of the laser light by the fungus, in which case the authors are justified in suggesting that it may be possible to “optimize the effectiveness of stain removal by selecting a laser wavelength that is strongly absorbed by the stain.”

However, the authors go on to suggest that a selective absorption mechanism is consistent with their observations that the purple (sometimes referred to as pink) and brown stains were not removed by the 532 nm (green) laser light, while the green and black stains were removed with some efficiency. In support of this argument they state that “neither [the purple or brown] stain effectively absorbs the green light of the laser.” In fact, exactly the reverse is true. A purple material absorbs light in the green region of the visible spectrum (reflecting blue and red wavelengths) while a green material absorbs all colors except green. Therefore, an absorption mechanism alone would predict the green 532 nm light to be the most efficient at removing the purple stain and least efficient at removing the green stain, in contradiction with the authors' observations.

From the observations presented in the paper I might speculate that absorption is less dependent on the color than on the optical density (darkness) of the stain. The two stains observed to be most effectively cleaned by laser irradiation (Alternaria solani and Penicillium notatum) appear in figure 2 to be considerably darker than the other two, and therefore more likely to absorb light even of the “wrong color.”

Secondly, there are some apparent inconsistencies within the studies of the removal of the fungal bodies by laser irradiation. Figure 4 shows paper covered with Penicillium notatum spores treated with 532 nm radiation, while figure 6 shows Fusarium oxysporum covered paper treated with dye laser irradiation. The authors state that both treatments were successful in removing fungal bodies from the paper. However, in order to make a valid comparison of the relative effectiveness of removal of the fungal bodies, obviously the experiments should have been carried out using similar treatment methods.

The validity of comparison of these results breaks down even further if one considers the total energy delivered by each laser system. The Penicillium sample in figure 4 was exposed to 1,600 mJ of energy (assuming exposure to 10 pulses of 532 nm radiation at 160 mJ/pulse, calculated with the assumption that the quoted 1.6 watts average power output was measured at 10 Hz). By comparison, the Fusarium sample in figure 6b was exposed to 93,600 mJ of energy, having been subjected to the output of the dye laser for “3 minutes of pulsing at 20 pulses per second” (3,600 pulses of 26 mJ/pulse, again assuming the average power output was measured at 10 Mz). Clearly the great disparity between the amount of energy needed to remove the two different species of fungal bodies should be considered.

Furthermore, the size and homogeneity of the laser beam must also be considered. The spot size of an unfocused dye laser beam is typically much smaller than that of the unfocused beam from a Nd:YAG laser, and both lasers most likely have somewhat uneven spatial distributions, giving rise to “hot spots.” Given that the Fusarium sample in figure 6 was subjected to a much greater amount of laser energy condensed into a smaller area than was the sample in figure 4, it is not unreasonable that the laser might have created small pinholes in the paper. The authors state that if this were the case “one would expect to see similar holes in fungus-free samples,” but do not explicitly state whether irradiation of a fungus-free sample under the same conditions as in figure 6 was performed to support this statement.

Finally, when specifying the energy output of any pulsed laser system, it is conventional to report the pulse energy, in joules/pulse, along with the wavelength of the laser radiation. Watts, a measure of energy output per unit time (power), are reserved for specifying the output of continuous-wave laser systems. In this paper, the average power of the pulsed systems are reported in watts, and the mistake is compounded by the omission of the repetition rate at which the measurement was made, which would allow the reader to determine the pulse energy. Consequently, in order to make the comparison of energy exposure in the different experiments discussed above, an assumption as to the repetition rates of the laser had to be made. As a short aside, the typical pulse width of a Nd:YAG laser system is on the order of 10 nanoseconds (10 � 10-9 seconds). Thus, the true power output of the 532 nm laser (that is, considering only the time that the laser is actually emitting light) is 1.6 � 107 watts, 10 million times higher than the value of 1.6 watts reported.

AUTHORS' REPLY

Dr. Trentelman raises several questions about our work, and suggests the need for clarification of several points that we make. Let us address her concerns in the order in which she raises them.

1. Dr. Trentelman raises a question of the color of the stain and the color of the light absorbed. She is of course correct in her general argument that pigments absorb the complement of the color that they reflect, a point that is well known to both authors from artistic and spectroscopic perspectives. When we state that the laser light is absorbed by the pigmented stains that are removed, but not by those that are not removed, this is a statement of fact. We measured the absorption spectra of the stain in solution to determine the wavelength or color at which they absorb light. While the exact wavelength of light absorption can be different when a pigment is absorbed onto paper than when it is in solution, our measurements in several different solvents suggests that the absorption spectrum of these stains are not particularly sensitive to whether they are on paper or in solution.The source of the misunderstanding appears to lie in our imprecise description of the colors of the stains. Our naming of the colors is consistent with standard practice, even though the precise color depends upon conditions such as light, pH, and nutrients. Clearly the black stain of Alternaria solani absorbs light throughout the visible spectrum including the 532 nm light of the laser. The second stain that was produced by Penicillium notatum and removed by this wavelength of laser light was described as “light green.” It is really more of a muddy yellow as can be seen in figure 2. The colors of the other two stains, which were not removed by the laser, can hardly be described as spectrally pure in the sense that Dr. Trentelman's analysis would suggest. We regret any confusion over this point by our choice of conventional descriptive terminology.
2. Dr. Trentelman “speculate(s) that absorption is less dependent on color than on the optical density (darkness) of the stain.” The critical point is that the amount of laser light absorbed depends upon the optical density of the stain at the wavelength (color) of the laser. Hence it does depend on the color of the stain which may not absorb all wavelengths of light. The fact that we were in two cases able to remove stains completely, means that at some point during the irradiation, the optical density of the stain became very low, just before the stain disappeared. In the two other cases where the stain did not absorb appreciably at the wavelength of the laser, we were unsuccessful in removing the stain despite the fact that the stains appear quite dark (see fig. 2). Hence optical density alone cannot account for removal success.
3. Dr. Trentelman raises questions about the removal of fungal bodies by laser irradiation. First the scanning electron micrographs, figures 4 and 6, are unambiguous. The fungal bodies are removed by laser treatment. We ascribe the small fungal filament size 5–10 μm holes that we see following the removal of the Fusarium filaments to the cavities left when these bodies were removed. We rejected the alternative likely possibility that the holes arose from laser damage to the paper by stating that otherwise “one would expect to see similar holes in fungus free samples.” We are criticized for not explicitly stating whether irradiation of fungus free samples was conducted under the same conditions without producing holes. In fact it was our intent to convey exactly that evidence when we made the statement quoted above.We perhaps should have followed it with an additional statement that explained that we had in fact carried out such a control. We might add that if there was damage to the paper by the laser, it would most likely show up as alteration in the structure of the cellulose fibers rather than as neat round voids between the fibers.A comment was also made about the size and homogeneity of the laser beam, and of the possibility of hot spots that might create pin holes in the paper. Certainly bright “hot” spots are possible in a laser beam, but as shown in figure 7, and described in the text, single YAG laser pulses produced a roughly circular clean spot approximately 4 mm in diameter. No micron scale holes were observed except when the lower power dye laser was used on the Fusarium filament type fungus. It therefore seems unlikely that laser hot spots could be the cause of the holes.
4. It is also pointed out that we reported the removal of two different fungi using different types of lasers operating at two different power levels, and “in order to make a valid comparison … obviously the experiments should have been carried out using similar treatment methods.” While we would agree that much more work can be done, we were simply reporting our findings that fungal bodies were removed using both kinds of lasers to suggest the range of possibilities of LSR.
5. Finally, in two places considerable attention is given to the actual power levels of the lasers utilized. Dr. Trentleman's analysis is absolutely correct. In point of fact, a pulsed laser has a very low duty cycle. That is, it is really off most of the time, and all of the energy is released in short bursts. She correctly points out that for our laser the bursts are only 10 nanoseconds (billionths of a second) long, making the peak power more like 16 million watts rather than the 1.6 watts of average power we reported. Our reason for giving the average power at 10 pulses per second was that it is a much simpler quantity to measure, and is proportional to the peak power of each pulse. As a practical matter, when carrying out laser stain removal, a conservator, would most likely utilize a simple average power meter to monitor the output of the laser and determine basic operating conditions.We hope that these responses are helpful responses to the questions posed by Dr. Trentelman, and that they clarify our original paper.

1.1 LETTERS WELCOMED

The editor welcomes letters concerning the specific contents of papers published in JAIC. Letters accepted for publication will be printed in whole or in part at the editor's discretion.