KEN BARAT, LASER SAFETY SOLUTIONS
As distinguished readers of Photonics Spectra, you may feel as if there is little more to learn about laser protective eyewear, though certain core knowledge is worth repeating, and there may be items of which some readers are not aware.
Did you know, for instance, laser protective eyewear is only required to have the same impact resistance as commercial sunglasses?
There are several standards that
address laser protective eyewear as well as a number of requirements that manufacturers must meet. “Requirements” may be too strong of a word. The testing of many of these items is guided more by good practice than mandated requirements. For example, eyewear is not
supposed to continue to burn for >1.5 s when it is exposed to a Bunsen burner flame.
When a laser user or laser safety officer selects protective eyewear, there are, however, several critical factors that need to be satisfied. First the selected filter must absorb the laser wavelengths to the extent that any wavelengths penetrating the filter are at or below the maximum permissible exposure value. This applies even to alignment eyewear, meaning the filter must block the wavelengths of concern and be at the proper optical density.
Image courtesy of iStock.com/M?xa?? Py?ehko.
Further, the eyewear frames must fit the user’s face and any prescription eyeglasses without sliding down the nose. For some eyewear and eyewear users, this also means that there cannot be gaps on the side of the frames, while for others this is a minor concern.
I am only aware of two incidents in which beams entered through side gaps. In neither case did the beam enter the pupil.
Practical vision is another important requirement for safety eyewear. If users cannot adequately see through the filter,
it is unlikely that they will use the eyewear as designed. Therefore, visual light transmittance (VLT), or perceived VLT, must be addressed (See “Visual Light Transmittance Is Not So Transparent,” Photonics Spectra, February 2023).
A frequently overlooked consideration when selecting eyewear is the typical illumination level within the laser use area. Is it well lit? Or is there little to no lighting? This factor directly affects VLT and general work conditions. But it also demonstrates that filter selection cannot be conducted in a vacuum. One needs more information than the laser’s specifications.
If these items are addressed, then users can be confident in their laser protective eyewear choice.
The only regulatory requirement concerns product labeling. Laser eyewear must be properly labeled so users can assure themselves that they have the correct eyewear on. In the U.S., labeling encompasses optical density and wavelength. Under the International Electrotechnical Commission (IEC), labeling communicates the eyewear’s effective wavelength and scale number. Scale number is calculated by the IEC through a combination of optical density and the filter’s ability to withstand a certain irradiance at the optical density level.
Eyewear that is not properly labeled is illegal to use and an easy target during inspector audits. Though the illegible font size of eyewear labeling is a common complaint of end
users, there is no real
guidance for this. ANSI Z136.7 standards for Testing and Labeling of Laser Protective Equipment states that eyewear must be clearly marked, and the manufacturer must use the largest font size possible without impacting the user’s vision if it’s printed on the lens. This means most of us will struggle to read what is printed or etched on the lens or frames. The Z136.7 standard covers laser barriers in addition to eyewear but not protective clothing.
The eyewear checklist
Laser protective eyewear needs to meet only the ANSI Z80.1 standard for the
impact resistance that applies to sunglasses. This dates to a compromise on early laser eyewear when it was all manufactured with Schott glass. Not every plastic lens meets ANSI Z87.1. If impact resistance is important, users should check the eyewear carefully and not assume all plastic lenses meet the Z87.1 impact standard.
Eyewear that is not properly labeled is illegal to use, and is an easy target during inspector audits.
Angle sensitivity is also on the checklist for laser-protective eyewear. It is best to have straight-on exposure. Angle sensitivity that is <45° from the center line, however, can affect the frame’s listed optical density. This changes as the angle of exposure to the filter becomes more obtuse, which is particularly true with dielectric coated filters.
The so-called 10% rule applies to a number of environmental factors. The
optical density and luminous transmittance are not supposed to change by >10% due to environmental conditions, including humidity, use in high or low temperatures, or rapid temperature changes (i.e., temperature shock). The issue of humidity most commonly applies to eyewear that is used in medical settings in which it might be exposed to sterilization devices.
Solarization is an effect in eyewear that results from exposure to UV radiation from either manufactured sources or sunlight. While most laser labs routinely cover exterior windows, if any exist, the use of laser systems outdoors or in well-lit factory settings is increasing. Therefore, awareness of solarization becomes more important for laser eyewear. Over time, UV radiation will degrade polycarbonate. I have observed VLT degrade in laser protective eyewear. But, in all those cases, the eyewear was used for more than 10 years. Solarization can also lower the originally labeled optical density level. The standard most commonly applied to solarization testing is MIL-STD-810H.
One final word of caution: Laser protective eyewear needs to be cleaned on occasion, and every end user needs to know how to perform this task. This doesn’t pertain to the cleaning technique that is employed so much as which chemical solutions are safe for the eyewear. Certain cleaning solvents can adversely affect coatings on the filter.
These guidelines don’t signify the end of testing and parameters that apply to laser protective eyewear. To some degree, meeting these testing levels explains the cost of such eyewear. So next time you handle your eyewear, be gentle and
appreciate all they have gone through to be equipped to protect you.