Too often, a laser user's first view of laser safety consists of maximum permissible exposure limits, optical densities, L Ratings, Nominal Hazard Zones, laser safety officers, and beam divergence. Available evidence suggests, however, that non-beam hazards and workplace habits should be of at least equal concern to the supervisors of laser installations. From incident data collected during the past 30-plus years, it is apparent that human error is a critical factor in unsafe laser use (see figure 1).
Figure 1. Improper usage causes most laser-based injuries.
We are often surprised at the lack of laser knowledge and awareness exhibited by laser users. More surprising are the laser-safety solutions that create other hazards when not properly implemented. A collection of real-life situations encountered by our team should provide some practical and useful insights into laser safety.
Our aim is to provide laser safety officers and shop floor supervisors with clues to watch for when trying to ensure the safe use of lasers in industrial and research settings. From this work, we propose a laser-safety checklist to supplement existing regulations, industry standards, best practices, and laser manufacturers' recommendations. We also propose that laser-safety programs be more closely integrated with general workplace safety guidelines and organizations. THE EYES HAVE IT
A job-shop customer returned a pair of eyewear with a broken frame in hopes of having the product replaced in accordance with the warrantee offered at the time of sale. Several characteristics of this very worn pair of laser-safety glasses left a lasting impression. First, one of the temples had been stressed beyond the point at which the eyewear would remain safely in place under any circumstances. Second, the brow bar, which provides both support and additional protection, had been removed. Third, both of the glass lenses were spattered with tiny speckles of molten metal, leading us to wonder how close to the machine the operator was who depended on the eyewear to safeguard his or her vision.
Have you examined the safety eyewear issued to your workers? The condition of eyewear and other personal protective equipment can provide important indications of unsafe behavior in users. It is not just important that the eyewear be in usable and effective condition; if the equipment looks too good, you should question whether it is being used.
Recent advances in laser-safety eyewear have resulted in cost-effective designs that offer excellent, gap-free protection, lightweight polymer and glass constructions, and attractive styling. This variety virtually guarantees that the end user can find at least one model that fits his/her unique cranium and budget. We are dismayed by the number of environments driven by accounting into a "one size fits all" mentality. If forced down this path, the manager should try to ensure that a model is selected with both tilt-ratcheting and length-adjustable temples because these afford the best comfort; remember, no one will wear an uncomfortable item.
Select a model that fits the particular work environment. For example, impact resistance is a requirement that is easily overlooked when sourcing laser-safety eyewear. Also, consider the visible light transmission offered by the filtercan personnel work safely given the room-darkening effects created by most laser-safety eyewear? A less obvious characteristic is retention. We have heard complaints that frame designs with straight temples that do not wrap around the ear are not suitable for environments in which the laser operators face downward because the eyewear has a tendency to slip off of the head. We recommend the use of retention straps at all times because it is impossible to predict when something in the work area may cause the laser operator to suddenly shift his or her head position. Even the simple act of picking up a dropped tool can create a hazardous situation under the right circumstances.
Finally, consider the wavelengths that are attenuated by the filter. If the equipment operator needs to watch for a red light during the process, either his eyewear should transmit those wavelengths or the process indicator color should be changed. This type of problem is common in medical applications in which physicians wearing certain glasses cannot easily see veins and other skin characteristics. A HOLE IN THE WALL
Safety eyewear is not the only thing that can provide important clues to operator usage. Be sure to inspect walls, curtains, barriers, and enclosures for holes, burn marks, and other indications of misdirected laser energy. Whenever possible, laser beams should point downraw beams should not emanate away from the vertical plane. We have found burn holes and scorch marks in a number of installations. We have also observed untested or uncertified materials such as plywood and vinyl being used as beam stops. Some personnel viewed the burn marks in these materials as a sort of badge of courage.
Inspecting the entire perimeter of the lab or workspace with the personnel who work there provides an opportunity to discuss the safety of both the direct staff and any personnel in neighboring spaces. It is a time to offer important reminders of safe practices and dangerous shortcuts.
In the laser labs of one very highly regarded research institution, we found some of the best minds in the world working with skilled technicians in a state-of-the-art facility to develop new lasers and applications. We also found general clutter and miscellaneous obstructions, including cooling lines laid across the floors. Flaws in basic housekeeping introduced safety risks, despite the attention to protection from laser radiation.
When a young lab worker invited us, unprotected, into the proximity of an ongoing laser experiment, we realized that laser-safety equipment alone is not enough. Individual behaviors must be modified with training and reinforcement. The emphasis must be on overall safety, not just laser safety.
At least two corollaries follow from this call to pay attention to the condition of the work environment. First, mitigate the risks associated with assist gases like oxygen and other media that may be present alongside the laser system and that may create fire or explosion hazards if the laser beam acts on them. Second, evaluate the gas and particulate matter created by the laser process. Laser-generated airborne contaminants are evident in every laser process, without exception, and range from nuisance to extreme respiratory hazard (see table). BARRIER DESIGN
We have uncovered a host of issues concerning the safe design and installation of both fixed laser-safety-curtain systems and movable laser-safety barriers. At one extreme is the end user who installed heavy black paper in the windows of his laboratory, assuming that the amount of laser energy that would reach these surfaces under normal conditions was very diffuse and of low power. Assumptions are always dangerous, especially when safety practices are involved. There are many unplanned events that could create a hazard: a pen could fall out of a shirt pocket and inadvertently move a mirror, an elbow could bump against optics while the technician is making adjustments on his table, a loud noise or unexpected entry could startle the technician. Always plan for a reasonable worst case.
Laser curtain and barrier materials are available with ratings for low power, 100 W/cm2 for example, and even powers as high as 1200 W/cm2. We have received at least one inquiry for a barrier capable of blocking many thousands of watts. Such barriers can be designed and built, but we question the safety of live personnel in any space in which such an extreme amount of laser energy is likely to be presenteven wearing a suit of armor would not protect the laser user from potential burns. In cases such as this, appropriate Class 1 enclosures and beam tubes must be coupled with carefully articulated and monitored control procedures including interlocks, lock-out/tag-out procedures, visible and possibly audible control signals, and emergency-shutdown mechanisms.
Assuming the selection of proper barrier materials, overall design and installation becomes important. Be familiar with the other design criteria for laser-safety curtains and barriers (see figure 2). Minimize gaps, parts that wear, and reflective surfaces. This applies to the laser setup itself; for example, the nice smooth black anodized optic holders that are widely used in laser labs are specular reflectors in the near-IR spectral region. Black can be deceptive.
Figure 2. Laser barrier design criteria involve proper materials, assembly, and marking.
At one site, we found a very neat-looking laser enclosure fabricated with polymeric viewing windows inserted into an aluminum frame. The windows were tinted to a color that is common for laser-safe viewing acrylic; it was, however, an ordinary acrylic sheet from a local manufacturer, designed to provide dust protection only. We were told that the operators of the equipment within the enclosure always wore laser protective eyewear.
Fine for them, but what about the visitor who is unaware of the details? If the enclosure is not laser safe, it should not be designed to look laser safe. Further, locate appropriate warning labels and signs at the entrance to the room as well as on and near the enclosure.
A variation on this theme concerns the eyewear itself. When asked what types of lasers he was working with, a job-shop technician indicated that he was marking with both neodymium-doped yttrium aluminum garnet (Nd:YAG) and carbon dioxide (CO2) laser systems. When asked about personal protection, this same person remarked that he just grabs a pair of laser-safety glasses from the shelf and that he prefers a certain pair with clear lenses: "The clear ones are better because I can see better." It turned out that this hapless technician was operating his YAG marker while wearing CO2-only protection!
Although the supervisors at this company provided the laser-safety tools, they did not provide either training or control procedures that would ensure the proper use of the equipment. Users need to understand the equipment and understand the requirements to ensure their continued safety.
Lasers can seem mysteriously powerful but we are convinced that the user does not need a PhD in physics to operate a laser system safely. A newly delivered laser system will come with emergency shutoff and interlockable circuits. The user manual will describe the recommended eyewear and how to set up a safe laser workplace. From that point, it falls to the end user to use basic common sense and follow well-known industrial safety guidelines.
It is important to identify and train a laser safety officer who can evaluate beam and non-beam hazards. The solutions offered by the laser safety officer, however, must be integral to the site's overall safety program. Personnel need to be trained, awareness levels need to be maintained, and inspections and walk-throughs must be routine. We are, after all, only human. oe
Thomas MacMullin is president and general manager at Kentek Corp., Pittsfield, NH.