Laser Eye Protection

BACKGROUND

Selecting the right LEP depends on the wavelength of your laser and the optical density requirement.

Wavelength. Wavelength refers to the size of the waves and is related to the frequency. Visible wavelengths are associated with a color (i.e. different wavelengths of visible light will be visible as different colors). Visible wavelengths of light are typically defined as light between 400 and 700 nanometers (nm). Wavelengths from 200 nm to 400 nm are defined as ultraviolet (UV), and wavelengths from 700 nm to 1000 micrometers (mm) are defined as infrared (IR). It is important to remember that the human eye is most sensitive to visible light, and some sources of UV and IR radiation may pose a hazard even if they are not visibly bright.  

Optical Density. The optical density (OD) is a logarithmic measure of how much light is attenuated by a filter (or, in this case, attenuated by LEP). The larger the OD value, the more light that is blocked. Since the scale is not linear, small changes in the values of OD lead to a large change in the stopping power (see Table 1). For example, eye protection with an OD of 3.0 has 100 times more light blocking ability than eye protection with an OD of 1.0 (i.e. light will be reduced 100 times more when eye protection OD 3.0 is compared to eye protection with an OD of 1.0).

Please note that, for example, even though a pair of LEP with an OD of 6.0 only has a transmission of 0.0001%, that still might not be enough attenuation to reduce a laser exposure to safe levels! The OD requirements are always specific to each laser. Always use the recommended OD in the APHC NRD study, or the manufacturer's recommendation for non-military specific lasers. The class of laser has no bearing on selecting the proper LEP, although LEP is not usually recommended for Class 1 or Class 2 lasers. 

SELECTING LASER EYE PROTECTION

Most LEP will have a range or ranges of wavelength with an associated OD for each range. You will see this most often etched onto the lens or arms of the glasses or goggles. An example of the etching might be: "400-550 nm OD 6+, 600-700 nm OD 4+." The example means between 400 and 550 nm, the LEP will provide an OD of 6.0. It will also provide protection between 600 to 700 nm with an OD of 4.0. This means you should only use this particular pair of LEP at those wavelengths if the OD is high enough to meet the NRD recommendation (or manufacturer recommendation for non-military specific lasers). Any LEP which matches the wavelength output of your laser and meets or exceeds the OD recommendation should provide sufficient coverage in the event of a laser exposure. Some lasers will emit multiple wavelengths simultaneously, or can be tuned to different wavelengths. For these lasers, you will need to swap LEP as you change wavelengths, or find LEP that will provide coverage for every wavelength you will be using. Some LEP that blocks visible light may be difficult to see through when the OD is high. LEP with reduced visible light may create hazards of their own since the wearer may be unable to see tripping (or other) hazards. Only use LEP that allows sufficient vision to perform work tasks safely, and does not interfere with colored indicators (such as warning lights).

Absorption curves for LEP are not linear, and protection against a certain wavelength range does not imply protection of nearby wavelengths (i.e. if your laser emits just outside of the LEP wavelength range). Ensure that the selected LEP provides coverage for the entire wavelength range of the system in use. If eye protection is available but the optical radiation protection coverage is unknown because no markings are present, an OD measurement of the LEP using a spectrophotometer may be required (which the APHC NRD may assist with). The user may want to just replace the eyewear if the wavelength or OD is not known. Optical devices  not marked  with  the  level  of  protection  at  the  laser  wavelength  should  be  assumed  to  offer  no  protection  unless  verified.

Complete spectra of LEP should be available from the manufacturer of the LEP and will show the OD at wavelengths found outside those indicated by the etchings. Below in Figure 1, a spectrum of the optical density vs wavelength of a pair of sample LEP glasses is provided to give the reader an indication of possible protection levels. Every pair of LEP will provide different levels of protection, so it is important to ensure adequate protection will be provided based on the laser sources used by you.

Figure 1. Sample Absorption Spectrum of a Pair of Laser Eye Protection Glasses. U.S. Army, APHC.

When a laser is purchased from a manufacturer, the proper LEP will usually be included with the laser, but these are sometimes lost or moved around. There are a variety of places to purchase LEP off-the-shelf, but they should be purchased from a reputable manufacturer to ensure the LEP will provide proper coverage. For military personnel, eye protection should be chosen from the Approved Protective Eyewear List (APEL) (https://www.peosoldier.army.mil/Equipment/Approved-Eyewear-QPL/). Some spectacles and goggles on the APEL have the ability to swap in laser protective filters.

LASER EYE PROTECTION TECHNOLOGIES

Absorption. The most common LEP uses absorption technology. This usually consists of a pigment specifically created to absorb the wavelengths of light of interest. This pigment might absorb some range of visible light, which will reduce the outward visibility slightly. The pigment used in absorptive eye protection can break down over time, usually due to UV exposure, so keep them stored away from sun (or other UV) exposure when possible. This may cause the LEP's protective quality to weaken, even though UV degradation of the pigment makes the LEP appear visibly darker. Deteriorated eye protection may also become brittle. Any eye protection with cracks or darkening should be replaced.

Reflective. Reflective, or dielectric coated, eye protection typically utilizes multiple layers of thin films of dielectric material. Certain wavelengths of light are reflected by the dielectric thin film, while others will pass right through. By stacking the thin films, a wide range of wavelength protection is possible and reflective eye protection typically has a higher visible light transmission than absorptive eye protection. Some reflective LEP have very specific wavelength ranges of protection, so ensure that your LEP provides adequate coverage for your laser output. Any reflective eye protection with damage to the surface of the lenses should be replaced.

Figure 2. Dielectric Laser Eye Protection.

Photo by Tech. Sgt. Christopher Boitz

United States Air Forces Central (https://www.dvidshub.net/image/1878905/aircrew-flight-equipment-enables-others-succeed).

FORM AND FUNCTION

Form and function of the LEP will ensure a tight and comfortable fit for the wearer. If you are constantly adjusting your LEP, or it is falling off your face, you will be more vulnerable to a laser exposure. As the saying goes, "the best PPE is the one you will wear." Some recommendations for ensuring proper fit are below.

Glasses. Laser protective glasses should protect the full front and the sides of the eyes. The sides maybe protected by a curved wraparound design which follows the facial features, or with side shields. Glasses should protect against direct exposure and reflections from a possible glancing angle. If glasses do not fit closely enough to the face, or slip, the eyes can be at risk of direct and reflected exposure. Consider using straps that attach to the arms and can be tightened to secure the glasses. Nose pads can also be useful for glasses that do not sit well on the nose bridge. It is important to consider how and where the glasses will be used (e.g., you may need tight-fitting goggles or glass when working in a confined space). If the glasses do not provide a good fit, consider using laser protective goggles instead. Multi-function glasses may offer additional protection beyond optical radiation such as protection from projectiles or chemical splashes.

Prescription LEP is also an option if you will be spending time working around one type of laser. This might be a more comfortable option

Goggles. Laser protective goggles may be more practical for some persons, especially those who wear prescription corrective glasses. While more difficult to take on and off, goggles typically provide a closer fit to the face than glasses, and provide coverage around the sides of the eyes. This makes them the best choice for reducing exposures from reflections. Goggle fit should be tightened as much as is comfortable for the user.

Storage and Maintenance

Storage and care of eye protection. Eye protection should be stored in the same room as the laser source or outside the door(s) to the area where the laser hazard may be accessible. Store LEP to minimize scratching and dirt or dust.

Eye protection should have as little exposure to UV as possible. They should be stored indoors in an opaque container if possible and in protective cases or sleeves. Any scratches on the surfaces can be detrimental to the performance of the protection. Eye protection should also be kept as clean as possible with extra care to prevent scratching to eye protection that utilizes reflective technologies. Cleaning should be done with dry, delicate task wipes or with a light amount of an alcohol based cleaner. Be sure to check with the manufacturer for guidance on cleaning your eye protection beforehand.

Other

Skin. The skin is also susceptible to damage from a laser. Tightly woven clothing will provide protection from many laser sources, but even if wearing tightly woven clothing, some powerful sources may still cause injuries and may even burn through clothing. The best way to prevent skin (and eye) injuries is to prevent any exposures using engineering and administrative controls. Clothing should cover any exposed skin to include the neck, arms, hands, and legs. Arm sleeves should be secured such that there is no exposed skin when arms are extended. Any clothing used as protection should also be fire-proof or fire-resistant if continuous exposure of a powerful source is expected. Selection of gloves should allow the user ample finger dexterity to perform their tasks without encumbrance. Be sure to inspect the gloves before use for any holes or cuts that would allow an exposure.

Room Safety. Curtains surrounding a worksite are an excellent way to reduce stray light. This prevents unintended exposure to neighboring workers and passers-by, and discourages onlookers from directly viewing the work being performed. Curtains should be flame-resistant and made of low-reflectivity materials. Light curtains may be a solution to preventing unauthorized access to the welding site. Light curtains consists of a sensor and transmitter placed on either side of the entrance to a work site and will shut down the system if the light is broken by someone or something passing between the sensor and transmitter.

Signage. Signage should be kept clean so that it is legible and posted in designated areas. The signs should also specify what kind of eye protection (or other PPE) should be worn.