January 2001 Volume 23 Number 1

Laser Safety Considerations

As the use of lasers as a tool in conservation grows, conservators are being given an increasing number of opportunities to see and use lasers. There are now several companies in the United States and Europe that are aggressively marketing lasers designed for conservation. Additionally, there are several groups of conservators who have been willing to demonstrate their use of lasers, on a limited basis, as part of their commitment to training and sharing information within the field.

While it is easy to argue that the responsibility for safety and the development of cleaning techniques lies with these groups, it is always best to approach these tools with some personal knowledge of the potential hazards to conservators as well as to artworks. The knowledge that someone should have warned of the dangers of using a particular system is of small consolation if one has experienced significant eye damage or damaged an artwork.

Often the first thing that one thinks of when addressing laser safety is the issue of eye damage. This is particularly important to conservators who derive much of their livelihood from their ability to see changes in artworks. A good understanding of the potential hazards associated with the use of lasers will allow conservators to insist on proper safety precautions and to remove themselves from dangerous situations.

The reason that a laser is potentially more dangerous than conventional light sources is that it generally produces a collimated beam of light. That implies that the beam will not spread into a large spot as it travels across a distance. If the beam intensity is not enough to harm the eye at the laser source (labeled a class 1 laser based on the ANSI standards of classifying lasers from class 1 to 4), then the laser is safe if you look directly into it either at the source or from across the room. If, on the other hand, the laser is stronger (class 2 or higher), exposure may produce a number of types of eye damage depending on the power and the wavelength of the beam.

Because the beam is collimated it does not matter if the viewer is very close to the source of the laser radiation or if he is a half a football field away. Getting hit in the eye with a high power collimated beam (class 3 or 4) will likely produce some form of serious eye damage. To mitigate this situation, laser beam delivery systems are often fitted with a lens that causes the beam to diverge into a larger spot over some distance. At what point the laser beam becomes harmless to the eye depends on the power of the beam and the power of the lens (rate at which it defocuses). With this design, if one turns up the laser power, the distance from the lens that is required to make a laser beam safe increases.

Since these calculations are hard to make on the fly, it is very unwise, and potentially illegal, to use any laser of class 3 or 4 without enclosing the work area and requiring the use of laser safety glasses (which are specific to the laser wavelength) by all people in the area.

There are three parts of the eye that are normally vulnerable to damage if exposed to laser radiation (cornea, retina, and lens). In many cases the wavelength of the laser light determines the type of damage that will ensue. The most commonly described damage occurs with a visible light laser (400nm to 750nm). In this case, the eye recognizes that there is light, and focuses the beam onto the back of the eye where the retina may be burned. This can cause permanent loss of sight over a large area of the eye. While it is not a pleasant story to repeat, it is worth noting that people experiencing exposures to high power visible laser light have reported that they heard a pop and that the last thing they saw was their own eye filling with blood.

In the case of non-visible laser light (e.g. infrared YAG lasers or ultraviolet Excimer lasers) the eye does not detect the light and so it does not try to focus it. Also the light tends to be absorbed in the cornea or the lens of the eye. For these reasons the intensity of light required to cause damage is usually higher (the eye is not focusing it to a small more intense spot), and the damage area is often the front part of the eye.

A powerful laser operating in some parts of the non-visible range will cause an effect similar to an uncontrolled version of modern laser surgery for correcting vision. With moderately powered lasers it is less likely that the damage will be catastrophic (unless the beam is focused to a small spot by the beam delivery system and hits the eye at the system lens' focal point). However, it is still not an experiment that I would advise one perform on oneself or others. Leave the eye surgery to the medical doctors. There is no reason for exposing a human eye to invisible laser light radiation during a conservation treatment. Furthermore, appropriate eye protection for many of these wavelengths is comparatively inexpensive and colorless.

Lasers that produce a beam that is in the UV or IR have another disadvantage. That is that one can't sense exposure to nonvisible light until damage to the eye is significant. If a laser is inadvertently left on or pointed in the direction of an unsuspecting person, he or she may be completely unaware of the problem, and the natural blink response is not triggered.Therefore, while the type of eye damage encountered may be different in the case of non-visable light, lasers emitting at these wavelengths are still ranked as class 3 and 4, requiring an enclosed work area and appropriate safety glasses.

Of course there are other potential hazards associated with using lasers. In addition to producing a powerful light beam, many laser systems charge up very large capacitors. The danger of electrocution is great if untrained operators attempt to repair or even inspect the power supply without properly discharging it. This is a potentially fatal mistake, and the hazard should not be underestimated.

Finally, with regard to operator safety, the material being removed by the laser does not just disappear. In most cases the material is vaporized into the surrounding air. If there is no extraction system (e.g. vacuum trunk), then the operator is being exposed to inhalation of the vaporized material. In this case the hazard depends on what is being removed by the laser.

The issues involved in avoiding alteration or damage to art by lasers are actually much more complicated than those involved in avoiding eye damage. While human eyes are all similar, art varies considerably in material construction and in the types of materials removed when cleaning with a laser. Still, many of the same concepts which apply to laser eye safety apply to the potential problems that arise during laser desorption of materials from the surface of art.

Any given material will absorb light at some wavelengths while reflecting others. Moreover some wavelengths may be transmitted through to subsequent layers depending on the composition and thickness of the topmost layers. At any point where the light is absorbed a certain amount will be converted to heat and electrochemical bond breaking.

For all practical purposes all laser ablation has a thermal component, although different wavelengths may produce more or less thermal effects. Additionally other complex phenomena such as plasma formation, phonon formation, acoustic wave formation, and expansion of the volatilized materials may occur. The only sure way of ascertaining the results of laser ablation for a given material is with careful analysis of the particular situation. As with traditional methods of cleaning, precleaning tests and analysis of past successes or failures can greatly enhance the likelihood of success when using lasers for treatment.

Conservators are well versed in the hazards and benefits of using solvents, acids, scalpels, and abrasives. The same careful understanding of the effects of laser treatments will ensure that both the conservator and the art emerge from the treatment experience better for it. In all cases, it is vital that one insists on developing a feeling for the safety issues when first encountering any laser system. For more information on laser safety, the Laser Institute of America offers inexpensive books and videotapes for sale. The fourth biannual LACONA conference will be held in Paris in the fall of 2001. Like previous LACONA conferences, it should be a good source of information on the practical applications of lasers in art conservation.

Lasers are rapidly becoming an accepted part of the conservation tool kit. Undoubtedly, there will be even more valuable uses for them in conservation in the near future. It is important that safety is not be ignored in the rush to develop these new applications. Too many conservators already suffer from chemical sensitivity, and too much art has been damaged by unsound treatments. Careful thought about all of the effects of lasers is best for the advancement of the technique and best for the health and safety of the art and the art conservator.