Walking the Knife Edge

From oemagazine June 2003
01 June 2003
By Catherine Scogin and Ken Barat

One of the most common tools of laser safety is laser protective eyewear. Anyone who works with lasers and doesn't wear protective goggles is walking the knife-edge of risk. Laser eyewear is only as effective as its frequency of use, though, and it will only be used if it performs and is convenient and practical to wear.

While the function of eyewear is clear to many, selecting the right pair for a specific laser application is nontrivial. In choosing laser protective eyewear, two important factors are wavelength coverage and optical density (OD). Unfortunately, some users look no further than these two parameters. In reality, a number of additional factors should affect your selection of just the right pair.

The goal of laser eyewear is that if laser radiation strikes the lens portion of the eyewear, the material will completely block or reduce any transmitted radiation to below the maximum permissible exposure level. This filtration or protection level is the OD.

For UV and IR radiation, the OD selected should offer full protection. In contrast, at visible wavelengths it is common to select less than full-protection OD to allow the user to see the beam for alignment purposes or beam manipulation. Beam visibility becomes a challenge when beam output is less than 100 mW or the beam is expanded. The temptation for users is often to either remove or look over their eyewear to see the beam. This is a risky action when one exposure can be all it takes to incur serious eye damage.

There is only one laser alignment standard for eyewear in the world, and this is the EN208 standard originating in Germany, which has subsequently been incorporated into the CE standard for Europe (see table).

optical issues

One of the challenges in laser protective eyewear is blocking out the wavelengths of concern while still allowing sufficient visible light to pass through for good vision. In the case of multi-wavelength goggles, the percentage of visible light transmission (VLT) can range from 4% to 30%, which may not be sufficient. VLTs below 20% in a well-lit room can still yield a dark field of view. If the room lighting is dim for experimental or process reasons, even a 30% VLT may present safety issues, pushing the needs for stronger engineering controls or local task lighting. Knowing the lighting conditions in which the eyewear will be used is crucial to effectiveness.

Lifetime is another issue to consider: how long will the filter last when exposed to laser radiation? If a 100-W solid-state laser requires eyewear with an OD of 5, then that eyewear should be capable of withstanding exposure to 100 W indefinitely. Manufacturers can provide information on the irradiance level the eyewear will withstand. This can help you decide between coated dielectric, glass, or polymer eyewear.

Testing by the Army branch at Brooks Air Force Base (San Antonio, TX) has shown that standard laser eyewear exposed to ultrafast pulses suffers from a non-uniform bleaching effect related to the relaxation time of the absorption molecules. Not all eyewear for ultrafast pulses demonstrates this effect, but a large enough proportion does to make it a real safety concern. Ultrafast laser users who want full protection will need to check with the eyewear manufacturer to verify suitability of the eyewear for their use. Usually, the manufacturer can provide a sample piece of the lens for testing.

Those working with lasers often use sensor cards to locate UV or IR beams. When a beam strikes the card, the material produces fluorescence in the visible spectrum. Users must ensure that the selected eyewear will allow visibility of that luminescence or glow while blocking the laser radiation. Making the user aware of applications and the extent of eyewear filtration is extremely important.

pragmatic issues

Practicality can be just as important as performance when it comes to laser-safety goggles. Eyewear that remains on the optical bench or hung on the wall is of no use, so it's important to find versions that are usable and comfortable.

Finding a single pair of protective goggles that will cover one or two wavelengths is generally a simple matter. The more wavelengths the eyewear must protect against, the the darker the eyewear typically gets. Alternatives can include simply using multiple pairs of goggles or going with a flip-down lens to add additional spectral coverage.

The importance of fit is often underestimated. Improper fit of protective eyewear not only impairs performance but increases the likelihood that users will set it aside and trust in luck. Users do not want to be constantly reminded they are wearing protective gear by eyewear that is too loose, too tight, too heavy, or fogs up or slips. Thus, the effort placed in finding proper-fitting eyewear is well worth the time. One size does not fit all. For loose-fitting eyewear, one solution may be to place a strap across the back to keep the frame tight. For users who wear eyeglasses, flip-downs attached to their frames may provide a solution.

Manufacturers offer a range of size options, including new eyewear for slim faces and for very large faces. There are options for fitting different nasal profiles, including flat or low nasal profiles, and combinations for small faces with flat nasal profiles. Adjustable temple lengths are also helpful, as well as temples with gripping ends. Bayonet temples (the straighter temple) can help fit large faces.

Impact resistance may or may not be an issue. If impact resistance is necessary for a given application, users can obtain goggles that are compliant with the ANSI Z87 standard for safety eyewear (most polymer eyewear is compliant), wear safety glasses over the goggles, or have glass laser eyewear hardened to meet Z87. The second choice can affect general vision and the comfort or ease of wearing the protective eyewear, while the third choice will affect the cost of the eyewear.

Those who work with lasers and need corrective eyewear used to find their choices restricted to Buddy Holly-style glasses. Prescription wearers now have several options, including eyewear with prescriptions ground into the glass laser lens, eyewear that holds prescription inserts, and eyewear with flip-downs that contain polymer prescriptions in the base or the flip. For ground lenses, the frame selections have widened to include titanium frames and frames with adjustable temples.

Weight of eyewear can be a particular concern in the case of multi-wavelength or prescription eyewear. Depending on wavelength combination, glass lenses 7-mm thick are not unheard of. This is two to three times that of normal prescription eyewear and may prove too uncomfortable for a user to wear for extended periods. As always, the concern is that discomfort will lead users to set the eyewear aside. Breakthroughs in polycarbonate prescription flips and over-glasses may help address this problem.

ANSI Z136.1 requires laser protective eyewear to be labeled with the wavelength and OD for which it is intended. The laser eyewear manufacturer will imprint on the eyewear the most common range of wavelengths and OD for a particular pair. For the vast number of laser users, this is satisfactory. A small segment of owners may use the eyewear for wavelengths not listed. Curves and other documentation provided by the eyewear manufacturer or distributor will show the OD at the desired wavelength.

Another crucial consideration in eyewear is whether it offers clear peripheral vision. Anti-fog capabilities can be important, especially for goggles. Laser-inscribed markings (printed ones wash off when cleaned) also help the longevity of the eyewear, and UV inhibitors help prevent polymer lenses from darkening over time. Finally, while cost is important, vision is priceless.

Depending on the application environment and human factors, there is more to eyewear selection than OD and wavelength. It is important to be aware of these factors when selecting eyewear. Remember, the only truly effective laser safety eyewear is the eyewear that gets worn. oe

This material was presented in its original form at the International Laser Safety Conference (Jacksonville, FL). For more information, see www.laserinstitute.org/conferences/ilsc2003.

Preparation is everything

The emphasis in laser safety is on prevention, but it's also important to prepare for the worst. Rapid response is critical. Every laser laboratory should include a poster that provides the names and phone numbers of emergency personnel and steps to be followed until help arrives. The number to call during off hours should also be listed. The poster should be reviewed as part of annual laser safety audits for accuracy and to update contact numbers.

If an incident occurs:

  • Keep the individual calm, preferably seated or lying down; avoiding panic or shock is the main goal.
  • If your institution has a central number for emergencies, call that number.
  • If you have a medical clinic on site, its staff should be called or notified by your response office. If medical or security is called first, they should have standing instructions to contact the laser safety officer. It is important that the medical facility have some understanding of laser eye injuries as well as the damage mechanism. The individual should receive the name of a retinal specialist for further evaluation or follow-up in cases involving visible or near-IR laser radiation—not all laser injuries have an immediate effect on vision. Consequently, initial and follow-up eye examinations are critical.
  • Transport the person to a medical facility. Many large facilities have a fire department or security force that would transport the individual; they should be instructed on how to handle a person with an eye injury.
  • Notify the individual's supervisor, the laser safety officer, and others working in the same area or on the same equipment.
  • Workers should discontinue using the equipment until an evaluation is conducted to see if a systematic error exists.
  • Once the facts of the incident are understood, a "Lessons Learned" notice should be written and distributed to the institution's laser-user community—not to assign blame but to point out the contributing factors and prevent recurrence.

—K. B.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract No. W-7405-ENG-48.

There's no such thing as careful enough

The U.S. Department of Energy has an extensive accident-reporting database called the Occurrence Reporting and Processing System. One incident involved a person with 15 years of experience with lasers.

An experiment was under way with a laser producing femtosecond, 1-mJ pulses at 500 Hz, with a beam size of several centimeters. The beam was aimed upward toward a periscope. The beam output was not lowered because it burned through the neutral density filters. In violation of written procedures, two researchers decided if they were careful, they could insert a mirror into the full-power beam path. As one researcher placed the mirror into the beam path, it reflected part of the beam into his eye. The person heard a popping sound and the eye subsequently swelled. The incident resulted in a 100-µm spot-size injury. The researcher's vision went from 20/50 to near blindness; the researcher still cannot read large print.

-K. B.

Catherine Scogin
Catherine Scogin is with GPT Glendale, Pleasanton, CA.

Ken Barat
Ken Barat is laser safety officer for the National Ignition Facility Programs, Lawrence Livermore National Laboratory; Livermore, CA.

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