Personnel Protection Equipment for Laser Safety

INTRODUCTION

The most common misconception laser users have about laser protective eyewear is that it’s the first line of defense against laser radiation. In reality it should be the last line of defense. Beam containment will do more for a laser user than laser protective eyewear. If you can eliminate the possibility of eye damage by enclosing the laser beam path such that no radiation exposure to the eye is possible, then do so. While critically important, the implementation of laser protective eyewear is always understood to be the second line of defense. Laser protective eyewear has a valuable role to play in laser safety and presents many challenges to the user and laser safety officer (LSO). This section covers the selection and use of laser protective eyewear.

Laser protective eyewear comes in two types: full attenuation and alignment eyewear. Full attenuation means that no visibility of the termination point of the beam (or inadvertent intrabeam exposure) is feasible when wearing the laser eyewear.

Conversely, alignment or partial attenuation laser eyewear allows an individual to see some of the termination point of the beam for various purposes such as beam collimation, laser beam path alignment, and so on.

Frequently, one encounters cases where LSOs recommend and researchers are then supplied with full attenuation laser eyewear that subsequently is underutilized because of research conditions where partial attenuation is required for proper execution of laser-related applications.

----2 FULL ATTENUATION

Without exception, for class 4 lasers and class 3b lasers (when the maximum permissible exposure [MPE] limit is exceeded), it’s recommended to provide full attenuation laser protective eyewear in all ultraviolet (UV) (i.e., nominal 190 to 380 nm) and ocular focus near infrared (NIR) nonvisible (i.e., nominal 700 to 1400 nm) wavelengths, as well as mid to far IR regions. The logic in doing this is that if one cannot see the beams and they exceed the MPE limits, then there is no reason to do anything other than fully attenuate those same wavelength regions.

Moreover, in the visible regime (i.e., nominal 400 to 700 nm) when detection of the termination point of the visible laser wavelength is not required for one's application, then full attenuation of these same visible wavelengths is also recommended.

The LSO has the task of recommending proper eyewear selection for the wavelength or wavelength region in question to meet the required optical density (OD) for each laser application ( Figure 8.1). Once the small source intrabeam OD for each laser wavelength or wavelength region has been calculated, various other ancillary conditions emerge that may positively (or negatively) impact the intended use of the chosen laser protective eyewear.

To be effective, laser eyewear must be worn. As obvious as this statement is, the single most prevalent cause - by far - of laser-related eye injuries is that laser protective eyewear, while is typically available and appropriate to the prevailing laser application, was not worn.

Why? This is where many of the ancillary features such as weight considerations between glass and polycarbonate lenses, acceptable versus unacceptable visual light transmission (VLT), subjective preferences of comfort and fit, prescription lens (Rx) capabilities, propensity of eyewear to fogging, and peripheral visual capacity or lack thereof come into play.

Above: Fig.1 Laser eyewear, Glendale laser products.

----3 VISUAL LIGHT TRANSMISSION

Undoubtedly, VLT and fit are the two most compelling factors in the usage or aversion to usage of laser eyewear. Simply stated, VLT is the mean average percentage of the entire visible spectrum, as weighted for blue spectral responsiveness, that is not filtered by these lenses. Repeatedly, experience has indicated that the higher the VLT, the higher the likelihood of eyewear usage and consequently laser eyewear safety compliance.

In many research and academic circumstances, overhead room lights may be turned off for a variety (e.g., beam collimation, alignment, etc.) of conditions, and VLT in these circumstances becomes a pre-eminent concern. Moreover, laser related electrical hazards, which have caused serious injuries possibly including death, must be fully considered in light of diminished visual acuity resulting from a loss of VLT when wearing laser protective eyewear. Lest we forget, while laser radiation can blind you, electricity can kill you.

Additionally, the distinction between OD and VLT, especially in full- attenuation conditions, is sometimes misunderstood or misrepresented. Assumptions abound that a higher OD necessarily implies a reduction of VLT. However, reduction of VLT is directly correlated to a higher OD only when visually limiting optical densities are directly attributable to the visible (i.e., nominal 400 to 700 nm) region only.

In laser eyewear attenuation conditions in UV; near, mid, and far IR regions;

or a multiwavelength combination thereof, one may encounter eyewear that posseses high OD, low VLT; high OD, high VLT; low OD, low VLT; or low OD, high VLT. In my estimation, any eyewear possessing a VLT at less than fifteen to twenty percent (15 to 20%) is dangerously close to creating a loss of visual acuity and causing other potential (notably electrical) dangers to become considerably more likely.

Therefore, in seeking full attenuation laser eyewear with appropriate OD values for one's application, increasing VLT may require certain trade-offs. Typically, this is the decision juncture at which one considers the use of plastic versus glass lenses. Polycarbonate lenses are lighter than glass lenses. As such, polycarbonate lenses have inherent (and perfectly logical) preferability to the user, especially in conditions where protracted usage is required. There are certain common and very prevalent wavelength regions (notably Nd:YAG at 1064 nm) in which glass lenses have higher VLT than polycarbonate lenses. In this instance, the trade off is that while one is increasing the VLT, one is simultaneously increasing the weight of the eyewear and thereby potentially diminishing its perceived comfort.

Fortunately, various manufacturers of both glass and polycarbonate eyewear have noted the general preference for polycarbonate eyewear and have made significant strides in increasing their products' VLT in Near Infrared Radiation (NIR) and certain other visible wavelength regions.

----4 COMFORT AND FIT

Comfort and fit considerations are the wholly subjective and depend entirely upon individual preferences that each wearer maintains concerning how a specific set of laser protective eyewear feels when worn. Comfort and fit primarily center upon personal preferences issues, such as overall comfort when evaluated in terms of short, moderate, or protracted wearing times. If a pair of protective eyewear does not fit properly, not only can it not perform its function to the required specifications, but the likelihood of it being used decreases. This is true for a respirator, facemask, and laser protective eyewear.

Users want their eyewear to be as natural an extension of their faces as possible.

They don’t want to be constantly reminded that they are wearing eyewear by its being too loose, too tight, or too heavy, fogging up, slipping, or other common problems. Therefore, finding properly fitting eyewear is well worth the time. One size does not fit all. One solution may be to place a strap across the back to keep the frame as tight as necessary. Another solution may be flip-downs on one's own glasses. Manufacturers offer a range of options in sizes, 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) also help in fitting large faces. Choices of laser protective eyewear have come a long way. All users should be able to find just that right pair.

----5 DAMAGE THRESHOLD CONSIDERATIONS

Once you find the appropriate eyewear with adequate OD to achieve full attenuation and suitable VLT, there is yet another trade-off to ponder, namely damage threshold considerations. As a general rule of thumb, polycarbonate eyewear can withstand approximately 100 W per cm^2 of direct incident laser radiation for approximately 10 seconds prior to damaging effects being noted on the lenses.

Glass eyewear can withstand approximately 10 times (~1000 W per cm^2 ) the value of polycarbonate laser eyewear for the same time duration.

With the assumption that a collimated, focused beam is impinging upon a discrete, nonwavering point on the polycarbonate or glass lens, polycarbonate lenses are prone to exhibiting sequentially a superheated plasma effect at the surface of the lens, degradation of the absorptive dyes (with possible carbonization and darkening effects noted), the emission of smoke, possible noxious odors, the emission of flame, and potential ultimate penetration of the lenses. Glass lenses are prone to catastrophic degradation effects where the accumulation of irradiant energy results in loss of integrity, with effects such as a popping sound when the beam strikes the glass lens, potential "spider vein" crazing and, with sufficient accumulation of energy, a complete shattering of the glass lens.

Generally speaking, these physical effects for both polycarbonate and glass lenses have readily apparent visual and auditory correlates that forewarn the wearer of an impending damage threshold danger. They come into consideration when one is deciding upon which trade-offs to implement in order to optimize the likelihood of eyewear suitability and will also be discussed when ultrafast pulse considerations are presented later in the section.

----5.1 SIDE SHIELDS

The ANSI Z136.1 standard in "Factors in Selecting Appropriate Eyewear" man dates users to "consider" sideshields. Overall, the presence of side shields is not an issue that can be considered and then decided against. Rather, even though they may impair peripheral vision, the presence of side shields is should be mandatory and commensurate with the level of optical density that the main viewing lenses provide.

The ANSI Z136.1 standard "Safe Use of Lasers" does not require laser protective eyewear to be ANSI Z87 compliant. ANSI Z87 is the standard for safety eyewear; the most common element is impact resistance. Therefore, in evaluating eyewear, the question of impact resistance needs to be addressed.

Is it needed or not? If it’s not needed, no further action is required; if the LSO hazard evaluation determines that it’s required, one has three choices:

1. Obtain a pair of laser eyewear that is compliant with Z87 (most polymer eyewear is compliant).

2. Wear safety glasses over the laser eyewear.

3. Have glass laser eyewear hardened to meet Z87.

Choice 2 can affect comfort or the ease of wearing the protective eyewear and general vision, while choice 3 affects the cost of the eyewear.

----6 PRESCRIPTIONS

Eyewear for prescription wearers has several options. These include eyewear with prescriptions ground into the glass laser lens, eyewear that holds prescription inserts, and eyewear with flips, with polymer prescriptions in the base or the flip.

For ground lenses, the frame selections have been expanded to include titanium frames and frames with adjustable temples.

----7 SENSOR CARD USE

Most common sensor cards are used to locate UV or infrared beams. The laser radiation of concern strikes the card and produces a photodynamic reaction that yields fluorescence in the visible spectrum. The users need to make sure the eyewear selected will allow visibility of that fluorescence or glow. Making the user aware of application uses and the extent of the filtration of the eyewear is extremely important.

----8 WEIGHT

Weight of eyewear is a particular concern when acquiring multiwavelength or prescription eyewear. Depending on wavelength combination, 7-mm-thick glass is not unheard of. This thickness of glass two to three times normal prescription eyewear may prove too uncomfortable for a user to wear for extended periods. This can lead to a lack of productivity or not wearing eye protection. Breakthroughs in polycarbonate prescription flips and over-glasses may help improve this problem.

----9 LABELING

ANSI Z136.1 and the International Electrotechnical Commission (IEC) require laser protective eyewear to be labeled with the wavelength and OD it’s intended for. The laser eyewear manufacturer imprints on the eyewear the most common range of wavelengths and OD for a particular pair ( Figure 8.2). For the vast number of laser users, this is satisfactory. A small segment of users use eyewear for wavelengths not listed on them. Curves and other documentation provided by the eyewear manufacturer or distributor show the OD at the desired wavelength.

To be compliant, the facility LSO has to label the eyewear or post the information where the eyewear is stored and have a way to identify which pair is which.

Above: Fig. 2 Labeling.

----10 ULTRAFAST LASERS

Testing at Brooks Air Force base has shown a nonuniform bleaching effect on standard laser eyewear against ultrafast pulses. This relates back to the relaxation time of the absorption molecules. Not all eyewear for ultrafast pulses demonstrates this effect, but a significant number makes it a real concern. Therefore, ultrafast laser users who want full protection need to check the manufacturer's testing results to verify suitability of the eyewear for their use. Usually, the manufacturer can provide a sample piece of the lens for testing with a power meter in the actual application to verify the appropriateness of the lens in question.

It’s imperative to recognize that with ultrafast lasers (particularly regeneratively amplified sources) there exists the potential for OD values to be compromised should femtosecond beam exposure to laser eyewear occur. Should temporary or permanent loss of OD (and commensurate exposure levels in excess of applicable MPE values) occur as a consequence of these conditions, obvious detrimental eye safety effects become possible. The core safety issue surrounding laser protective eyewear and femtosecond lasers is as follows: In certain ultrafast (femtosecond) operating conditions, saturable absorption effects with calculable losses in purported OD values of the femtosecond-subjected laser eyewear have been observed.

It’s the intention of ANSI committees involved in this matter that the under lying mechanisms of the degradation effects be investigated and, to the greatest extent possible, elucidated for everyone's general understanding.

----11 ADDITIONAL CONSIDERATIONS

Another important consideration is antifog capabilities, especially for goggles.

Multiwavelength operations bring out special questions, because the more wave lengths you try to remove with one pair of eyewear, the darker the eyewear typically gets. You can try flip options or more than one pair to alleviate this problem. Laser-inscribed markings (printed ones wash off when cleaned) also help increase the longevity of the eyewear, and UV inhibitors prevent darkening over time in polymer eyewear. Finally, cost is important, but you also must consider the cost of an eye.

----12 PARTIAL ATTENUATION (ALIGNMENT EYEWEAR)

In laser-related research applications, investigators frequently need to view the termination point of visible laser sources. These beam alignment conditions are also acknowledged to produce a notable number of laser-related eye injuries.

The purpose of alignment eyewear is to allow the user to see the beam while lowering the intensity of any beam that is transmitted through the user's eyewear to a class 2 level. To address this issue, a European norm recommends OD for alignment eyewear versus the output of lasers used.

Scale #OD Max Instantaneous Power/Continuous Wave laser (W) Maximum Energy for Pulsed Lasers (J)

R1 1-2 0.01 2 × 10- 6 R2 2-3 0.1 2 × 10- 5 R3 3-4 1.0 2 × 10- 4 R4 4-5 10 2 × 10- 3 R5 5-6 100 2 × 10- 2

Therefore, for alignment laser eyewear to be effectively utilized, preferentially all of the following conditions should be in place: administrative liability acknowledgment and acceptance of it, acknowledgment of potential hazards with the utilization of eyewear that does not protect against small source intrabeam or specularly reflected exposures, and collaborative agreement between the LSO and researcher of alignment eyewear safety protocols and appropriate alignment laser protective eyewear. Once these preliminary philosophical protocols are established, the implementation of alignment eyewear can proceed.

----13 ADDITIONAL PPE

----13.1 PROTECTIVE WINDOWS

Laser protective windows can be thought of as large pairs of laser protective eyewear. They should meet the same requirements as eyewear regarding OD labeling. These windows are made of acrylic filter material.

----13.2 LASER CURTAINS

Overall laser curtains and barriers are used as passive guards to enclose an area where class 3B or class 4 lasers are in use to protect against accidental exposure to the laser beam or for long-term blocking of laser radiation at lower power densities. Similarly, some settings using class 1 lasers, which are class 4 under service conditions, require a temporary control area. This temporary control area barrier is meant to provide a safety barrier while work continues during service.

A wide variety of options exist concerning the level of protection curtains can provide. Examples follow:

If one is up for self-testing or -certification of curtain material, many standard varieties of welding curtains provide adequate laser barrier protection. The cost difference between these materials and manufacturer-certified laser curtains could be a factor of 10 or more. One may first wish to consult with the organization's legal department prior to trying this economic approach.

Irradiated Area -- Power -- Density Protection Time

1 mm^2 3 MW/m^2 100 sec 500 mm^2

3 MW/m^2 20 sec 500 mm^2 1 MW/m^2

60 sec 500 mm^2

0.7 MW/m^2 100 sec