16
Laser Eyewear
Interesting Eyewear Fact
In 1962, Dr. Harold Straub of the U.S. Army Harry Diamond Laboratory, developed the first laser eye protector by installing a 2 × 4-inch, blue-green glass (Schott-type BG–18), filter plate in a standard acetylene welding goggle frame.
The most common misconception laser users have about laser-protective eyewear is that it is the first line of defense against laser radiation. In reality, it should be the last line of defense. Beam containment will do more of a laser user than laser-protective eyewear.
If you can eliminate the possibility of eye damage because of enclosing the laser beam path such that NO radiation exposure to the eye is possible than do so. Although 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 Officers (LSO). The remainder of this chapter will deal with the selection and use of laser-protective eyewear.
Laser-protective eyewear comes in two flavors, full attenuation and alignment eyewear. By full attenuation I mean this eyewear will completely block the transmission of a direct exposure laser beam from penetrating the eyewear. Conversely, alignment or partial attenuation allows an individual, while wearing laser eyewear, to have some visibility, which means some of the beams energy will pass through the laser-protective eyewear (Figure 16.1).
Figure 16.1
Laser eyewear.
Frequently, one encounters cases where an LSO recommends, and researchers are then supplied with full attenuation laser eyewear, which subsequently is underutilized because of research conditions where partial attention is required for the proper execution of laser-related applications.
To talk about laser-protective eyewear, one really needs to understand two terms: optical density (OD) and maximum permissible exposure (MPE). OD is a filtration factor and MPE is like the speed limit. Your eye can accept irradiance up to and including the MPE without damage. The higher the exposure or irradiance over the MPE the greater the damage until a threshold is reached (one you do not want to reach), where the damage itself moderates the energy and damage (Table 16.1).
OD is a parameter for specifying the attenuation afforded by a transmitting medium. It is in log units; therefore goggles with a transmission of 0.000001% can be described as having an OD of 8.0. Logarithmic expression of OD can be described as follows:
|
where Mi is the power of the incident beam and Mt the power of the transmitted beam.
Thus, a filter that attenuates a beam by a factor of 1,000 or 103 has an OD of 3, and one that attenuates a beam by 1,000,000 or 106 has an OD of 6. The OD of two highly absorbing filters stacked together is essentially the sum of two individual ODs. When optical aids are not used, the following relationship may be used when radiant exposure (H) and irradiance (E) are averaged over the limiting aperture for classification:
|
where the radiant exposure (H) or irradiance (E) is divided by the MPE.
When the entire beam could enter a person’s eye, with or without optical aids, the following relationship is used:
|
where AEL is the accessible emission limit (i.e., the MPE multiplied by the area of the limiting aperture) and 0 and Q0 are the radiant power or energy, respectively.
Table 16.1 Code Definitions
Testing Conditions for Laser Type | Typical Laser Type | Pulse Length (s) | Number of Pulses |
D | Continuous wave laser | 10 | 1 |
I | Pulsed laser | 10−4 to 10−1 | 100 |
R | Q Switch pulsed | 10−9 to 10−7 | 100 |
M | Mode-coupled pulse laser | >10−9 | 100 |
Types Of Laser Safety Eyewear
Glass
Glass laser eyewear is heavier and more costly than plastic, but it provides better visible light transmittance. There are two types of glass lenses, those with absorptive glass filters and those with reflective coatings. Reflective coatings can create specular reflections and the coating can scratch, minimizing the protection level of the eyewear.
Polycarbonate
Polycarbonate laser eyewear is lighter, less expensive, and offers higher impact resistance than glass, but allows less visible light transmittance.
Diffuse Viewing Only
As the name implies, diffuse viewing only (DVO) eyewear is to be used when there is a potential for exposure to diffuse reflections only. DVO eyewear may not provide protection from the direct beam or specular reflections.
Alignment Eyewear
Alignment eyewear may be used when aligning low power visible laser beams. Alignment eyewear transmits enough of the specified wavelength to be seen for alignment purposes, but not enough to cause damage to the eyes. Alignment eyewear cannot be used during operation of high-power or invisible beams and cannot be used with pulsed lasers.
Laser Safety Eyewear For Ultrafast (Femtosecond) Lasers
Temporary bleaching may occur from high peak irradiances from ultrafast laser pulses. Contact the manufacturer of the laser safety eyewear for test data to determine if the eyewear will provide adequate protection before using them.
Labeling Of Laser Safety Eyewear
Laser safety eyewear shall be labeled with the OD and the wavelength(s) the eyewear provides protection for. Additional labeling may be added for quick identification of eyewear in multiple laser laboratories.
Inspection And Cleaning Of Laser Safety Eyewear
Laser safety eyewear should be inspected periodically for the following:
Pitting, crazing, cracking, and discoloration of the attenuation material
Mechanical integrity of the frame
Light leaks
Coating damage
Follow manufacturers’ instructions when cleaning laser safety eyewear. Use care when cleaning eyewear to avoid damage to absorbing filters or reflecting surfaces.
Considerations In Choosing Laser-Protective Eyewear
The most important considerations for picking eyewear are listed below. There may be other considerations.
OD requirement of eyewear filters at laser output wavelength(s)
Comfort and fit of eyewear with no peepholes
Visible light transmission requirement and assessment of the effect of the eyewear on the ability to perform tasks while wearing the eyewear
Need for prescription glasses
The following factors are included to calculate the OD of the filter:
Largest laser power and/or pulse energy for which protection is required
Wavelength(s) of laser output
Exposure time criteria (e.g., 0.25, 10, 100, or 30,000 seconds)
Comfort and Fit
Comfort and fit is a personal preference. Consider overall comfort when evaluating in terms of short, moderate, or protracted wearing times. If a pair of protective eyewear fits poorly, it will not work properly. Moreover, the likelihood of its use decreases. This is true for a respirator, facemask, or laser-protective eyewear.
One size does not fit for all. Users do not want uncomfortable eyewear that is too loose, too tight, too heavy, fogs up, or slips. The effort spent in finding proper fitting eyewear is well worth the time.
To help with fitting loose eyewear, you may need to place a strap across the back to keep the frame tight if necessary. Another option is to use flip-down eyewear over a user’s own glasses so the eyewear is familiar. Manufacturers offer a range of options in sizes, including new eyewear for slim faces to 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 an usable pair.
Optical Density
Full Attenuation
Without exception, for Class 4 lasers and Class 3B lasers (when the MPE limit is exceeded) it is recommended to provide full attenuation laser-protective eyewear in: all UV (i.e., nominal 190–380 nm), ocular focus near IR nonvisible (i.e., nominal 700–1400 nm) wavelengths, as well as mid to far IR regions. The logic in doing so is quite simple, 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 400–700 nm) when the 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 recom mended.
The LSO is tasked with recommending proper eyewear selection for the wavelength or wavelength region in question to meet the required OD for each laser application(s).
Once the small source intrabeam OD for each laser wavelength or wavelength region has been posted, various other ancillary conditions emerge that may both positively (or negatively) impact the intended use of the chosen laser-protective eyewear.
To state the obvious, to be effective, laser eyewear MUST be worn. As readily apparent and obvious as that comment is, the single most prevalent cause—by far— of all laser-related eye injuries is the fact that laser-protective eyewear, although 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 luminous transmittance (VLT), subjective preferences of comfort and fit, prescription lenses (Rx) capability, propensity of eyewear to fogging, peripheral visual capacity or lack thereof, and so forth, come into play.
Visual Light Transmission
Undoubtedly, VLT and fit are the two most compelling features 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, which is NOT being filtered by these same 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 of pre-eminent concern. Moreover, laser-related electrical hazards, which have caused serious injuries and may include death, must be fully considered in light of diminished visual acuity due to a loss of VLT when wearing laser- protective eyewear. Lest we forget, although laser radiation can blind you, electricity can kill you.
In addition, the distinction between OD and VLT, especially in full attenuation conditions, are sometimes misunderstood or misrepresented. Assumptions abound that a higher OD necessarily implies a reduction of VLT. However, the reduction of VLT is directly correlated to a higher OD only when visually limiting ODs are directly attributable to the visible (i.e., nominal 400 nm–700 nm) region only.
In laser eyewear attenuation conditions in UV, near-, mid-, and far-IR regions, or a multiwavelength combination thereof, there are relative instances where one may encounter eyewear that possesses: high OD, low VLT; high OD, high VLT; low OD, low VLT; low OD, high VLT. In my estimation, any eyewear possessing a VLT at less than 15%–20% is dangerously close to creating a loss of visual acuity where other potential (notably electrical) dangers become considerably more likely.
Therefore, in seeking full attenuation laser eyewear with appropriate OD values for one’s application(s), 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 in weight than glass lenses. As such, polycarbonate lenses have inherent (and perfectly logical) preference for the user; especially in conditions where protracted usage is required. There are certain common and very prevalent wavelength regions (notably Nd:YAG @ 1064 nm) where glass lenses have higher VLT than polycarbonate lenses. In this instance, the trade-off is of course that although one is increasing the VLT, they are simultaneously increasing the weight of the eyewear and thereby potentially diminishing the perceived comfort of the eyewear.
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 IR and certain other visible wavelength regions. Much of this improved VLT is because of eyewear that obtains all or some of its OD through the reflection of the laser beam. Yes, sufficient energy can be reflected off the eyewear to injure someone else if conditions are right.
These requirements should be used to aid in the selection of appropriate eyewear/ materials. VLT shall be computed and shall be available for the light- (photopic) and dark- (scotopic) adapted eye (see American National Standards Institute [ANSI] Z136.7 standard). Please keep in mind that normally the minimum acceptable photopic luminous transmittance is approximately 20% for most applications. Sunglasses are typically 12%–18% photopic luminous transmittant. Normally, the minimum acceptable scotopic luminous transmittance is approximately 20% for unaided applications.
Adequate OD at the laser wavelength of interest shall be weighed with the need for adequate visible transmission. The minimum adequate acceptable photopic and scotopic VLT is approximately 20% for most applications. At VLT levels less than 20%, other nonbeam hazards may exist by virtue of diminished visual acuity.
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 like: overall comfort when evaluated in terms of short, moderate, or protracted wearing times.
Overall, if a pair of protective eyewear does not fit properly, it not only cannot perform its function to the required specifications but also likelihood of it being used decreases. This is true for a respirator, facemask, or laser-protective eyewear.
Users want their eyewear to be as natural an extension of their faces as possible. They do not want to be constantly reminder they are wearing eyewear by it being too loose, too tight, too heavy, fogging up, slipping, or other well-known problems.
Therefore, effort placed in finding proper fitting eyewear is well worth the time. One size does not fit all. One solution maybe to place a strap across the back to keep the frame tight is necessary. Another solution maybe is to flip-down 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.
Ultraviolet 200–266 NM Beams
Always wear gloves and long sleeves when aligning UV beams to prevent skin exposure. Skin exposure to lasers could lead to possible skin cancer.
Use CaF2 substrate for transmissive optics to prevent nonlinear absorption of red fluorescence with high-energy, high-power UV beams. Red fluorescence ultimately leads to permanent increase of optical transmission loss (brownish coloring).
Use fused silica substrate for reflective optics to reduce coating absorption.
Aluminum-coated gratings, even when coated against oxidation, will degrade rapidly when used for UV high-energy beams.
Remember: the lower the wavelength, the smaller the spot size for a given focal length lens/optic. When looking at beam profile on camera, ensure ALL harmonics are filtered out.
Ultrafast Optical Parametric Amplifier (166 NM to 20 µm) Beams
For NIR and IR beams, liquid crystals papers (from Thorlabs or Edmunds Optics) can be very helpful to detect the position of far IR beams, outside range of conventional beam viewers.
Do not be fooled by harmonic components.
800 NM BEAMS
No wavelength has been involved in more laser eye injuries in the past 10 years as the Ti:S 750–850 nm beam. The eyes lack perception of this wavelength band—less than 1% of these photons are perceived by the eye. A user may see a faint dot giving the false impression of low power.
For alignment of an 800 nm compressed beam (peak power), you can use a white bleached business card (while wearing eyewear) to see the SHG (second harmonic generation [blue color]) beam on the card.
When aligning compressed or very intense large diameter beams, use the SHG on a white business card to center the beam on alignment irises. Center the beam on the iris looking at the throughput beam (symmetrically clipped SHG blue beam).
When aligning small diameter beams, use an IR viewer to look at the concentric beam around the hole of the iris or use an orange card looking at the throughput beam.
Beware of the secondary lasing cavity caused by back reflections when introducing reflective surfaces in a pumped amplifier with flat (not Brewster) Ti:S crystals (valid for other type of gain medium).
ALWAYS use a MINIMUM number of mirrors to realign an amplifier.
White thin ceramic plates are useful for finding the beam. They are safe with both low- and high-power beams.
Case 1
A 26-year old male student aligned optics in a university chemistry research lab. He used a chirped pulse titanium–sapphire laser operating at 815 nm with 1.2 mJ pulse energy at 1 kHz. Each pulse was ≈200 ps. The student was not wearing protective eyewear.
The laser beam backscattered off the REAR SIDE of a mirror (about 1% of total) and caused a foveal retinal lesion with hemorrhage and blind spot in central vision.
A retinal eye exam was done and confirmed the laser damage. Protective eyewear could have prevented the injury.
Case 2
A postdoctoral employee received an eye exposure to spectral radiation from an 800 nm Class 4 laser beam. The extremely short pulse (100 fs) caused a 100-μm-diameter burn in the employee’s retina. The accident occurred shortly after a mirror was removed from its mount and replaced with a corner cube during a realignment procedure. Although the beam had been blocked during several previous steps in the alignment, it was not blocked in this case. The employee was exposed to laser radiation from the corner cube mount when he leaned down to check the height of the mount.
Neither the employee who was injured nor another employee who was working on the alignment was wearing the appropriate laser eye protection. The researcher may have underestimated the hazard because the visible portion of the 800 nm beam only represented 1%–2% of the beam.
Flash Lamp Yag High-Energy 532 NM Beams
Always align beams at LOW POWER (detune the Q-switch [QSW] timing instead of LAMP timing to reduce green).
Always verify the YAG beam profile PRIOR to sending it to a Ti:S crystal or other crystals. Hot spots will likely cause severe irreversible damages to the crystal lattice or the crystal coating. Dummy testing on sapphire crystals can be an inexpensive way to ensure integrity of the Ti:S when pumped.
White ceramic is the preferred permanent beam block material for YAG energetic beams.
Practical short-term beam blocks for YAG 10Hz green beam are white packing foams, which diffuse the green powerful beams temporarily during specific and approved alignment procedures.
DO NOT USE black anodized metal surface as beam blocks. Photographic burn paper or nondeveloped black photo paper can be used to visualize the beam quality. Make sure to put the paper into a clear plastic bag to avoid debris blasts and avoid over-exposure. Beware of plastic bag laser reflections. Using back burns can help maintain information contained in the burn mark.
Yag/Ylf High-Power 532/527 Nm Beams
Wear Lawrence Berkeley National Laboratory-approved alignment goggles that allow you to see a faint green beam. Goggles are very useful for avoiding burns during alignment.
Remember that high power high rep rate beams will ablate black anodization of most beam blocks, leaving residues onto optics nearby.
Damage Threshold Considerations
Once one finds the appropriate eyewear with adequate OD to achieve full attenuation and suitable VLT, there is yet another trade-off hurdle to ponder, namely damage threshold considerations. As a general rule of thumb, polycarbonate eye-wear can withstand approximately 100 W/cm2 of direct incident laser radiation for approximately 10 seconds duration prior to damaging effects noted on the lenses. Conversely, glass eyewear can withstand approximately 10 times (≈1000 W/cm2) the value of polycarbonate laser eyewear for the same time duration.
With the assumption that a collimated, focused beam is impinging upon a discreet, 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. Conversely, glass lenses are prone to catastrophic degradation effects where the accumulation of irradiant energy results in loss of integrity with effects noted as: a popping sound when the beam strikes the glass lens with 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. However, they do come into consideration when one is deciding upon which trade-offs to implement to optimize the likelihood of eyewear suitability and will also be discussed when ultrafast pulse considerations are presented later in the chapter.
Side Shields
The ANSI Z136.1 standard, in “Factors in Selecting Appropriate Eyewear” mandates one to consider side shields, 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, I am of a mind that the presence of side shields is mandatory and be commensurate with the level(s) 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 ones eye-wear needs, the question of impact resistance needs to be addressed. Simply, is it needed or not. If not, no further action is needed; if the LSO hazard evaluation is, yes it is required, one has three choices:
Obtain a pair of laser eyewear that is compliant with Z87 (most polymer eyewear are compliant).
Wear safety glasses over the laser eyewear.
Have glass laser eyewear hardened to meet Z87.
Choice 2 can affect comfort or the ease of wearing the protective eyewear and general vision, whereas choice 3 will affect the cost of the eyewear.
Prescriptions
Now 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 widened to include titanium frames and frames with adjustable temples.
Weight
Weight of eyewear is a particular concern in the consideration of acquiring multi-wavelength or prescription eyewear. Depending on wavelength combination 7 mm of glass is not unheard of. This thickness of glass, which is two to three times a normal prescription eyewear, may prove to be uncomfortable for a user to wear for extended periods. This can lead to a lack of productivity or times of no eye protection. Some breakthrough in polycarbonate prescription flips and over-glasses may help improve this item.
Labeling
Figure 16.2
Labeling.
ANSI Z136.1 and International Electrotechnical Commission require laser-protective eyewear to be labeled with the wavelength and OD it is intended for. The laser eyewear manufacturer will imprint on the eyewear the most common range of wavelengths and OD for a particular pair (Figure 16.2). For the vast number of laser users, this is satisfactory. Always remember the guarantee of protection is only made for the wavelengths imprinted on the eyewear frame. Even curves for the eyewear are just a generalization. Unless you know the lot number and have curves for that run, only the imprinted OD is guaranteed. A small segment of users are using the eyewear for wavelengths not listed on them. Curves and other documentation provided by the eyewear manufacturer or distributor will show the OD at the desired wavelength. To be compliant the facility LSO will have to label the eyewear or post the information where the eyewear is stored and have a way to identify which pair is which.
Ultrafast Lasers
Testing by the Army branch 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 demonstrate this effect, but a significant number do, which makes it a real concern. Therefore, ultrafast laser users who wish for full protection will need to check with the manufacturer of the eyewear for their 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 is imperative to recognize that if one is using ultrafast lasers (particularly regeneratively amplified sources) there exists the potential that OD values may be compromised should femtosecond (fs) beam exposure to one’s 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 plausible. The core safety issue surrounding laser-protective eyewear and femtosecond-lasers is as follows: in certain ultrafast (fs) operating conditions, saturable absorption effects with calculable losses in purported OD values of the femtosecond-subjected laser eyewear have been observed.
It is the intention of ANSI committees involved in this matter that the underlying mechanisms of the degradation effects so noted be investigated and, to the greatest extent possible, elucidated for everyone’s general understanding.
Additional Considerations
Other important consideration is antifog capabilities, especially for goggles. Multi-wavelength operations have special questions, as the more wavelengths you try to remove with one pair of eyewear, typically the darker the eyewear 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 the longevity of the eyewear, as well as UV inhibitors to prevent darkening over time in polymer eyewear. Finally, cost is important, but you also must consider what is the cost of an eye?
Saturable Absorption
Certain dyes used to absorb laser radiation may undergo saturable absorption (aka, induced transmittance or transient photobleaching) where the ability to absorb radiant energy decreases with increasing radiant exposure or peak irradiance. When this occurs, the OD may decrease providing less protection to the user. This has been reported for both glass and polycarbonate filters for certain pulsed lasers and is associated with high values of peak irradiance. Lasers evaluated were pulsed (Q-switched and ultrashort pulses) titanium sapphire and neodymium:YAG (1064 nm and 532 nm) lasers.
Angle of Exposure
On the basis of the composition of the laser-protective-eyewear filter the angle of exposure can have an effect on the effectiveness of the eyewear filter. Dielectric coatings on laser-protective eyewear are designed to deliver the labeled OD within a set angle of acceptance (similar to the acceptance angle of an optical fiber. Laser radiation incident upon the eyewear outside that angle will yield a different OD. The obliqueness of the angle may or may not limit the laser radiation entering the pupil.
Cleaning and Inspection
Periodic cleaning and inspection shall be performed on the protective eyewear to ensure they are maintained to a satisfactory condition. The frequency of the safety inspection should be once per year, or as determined by the LSO. This includes the following:
Periodic cleaning of laser eyewear. Care should be observed when cleaning lenses of protective eyewear to avoid damage to the absorbing and reflecting surfaces. In some uses (e.g., surgery) eyewear may require cleaning (and sterilization) after each use. Consult eyewear manufacturers for instructions for proper cleaning methods.
Inspection of the attenuation material for pitting, crazing, cracking, discol-oration, delamination or lifting of dielectric coatings, and so forth.
Inspection of the frame for mechanical integrity.
Inspection for light leaks and coating damage.
Inspection of goggles for loss of ventilation port plugs, deformation of the face piece, and stretching of the head strap.
Eyewear in suspicious condition should be tested for acceptability or discarded. If, upon inspection, the LSO is unsure of the severity of these defects, as they relate to efficacy of use, the LSO should contact the manufacturer for guidance and recommendations as to replacement or acceptability of current laser-protective eyewear usage.
Use With High-Power Lasers And Eyewear Limitations
The following section is from the ANSI Z136.8 standard for laser safety in research and development. The key in this section is that if you have a high power laser or a beam of very high irradiance, counting on laser-protective eyewear as one sole or primary safety control is foolish. You are saying “I hope if there is a stray reflection, it hits my eyewear and I can get out of the way before it burns through.” Engineering control measures shall be implemented with high-power, multikilowatt laser beams, unless impractical, control measures may be used. Personal protective equipment, in the form of laser eye protection, may be inadequate to protect the user from serious ocular exposure from such laser beams. In addition, if the multikilowatt laser beam does not strike the laser-protective eyewear, the skin of the face may receive a significant injury (e.g., third-degree burn and laceration from facial motion during exposure).
Most of the radiant energy absorbed by the filter is transformed into heat. If the radiant flux is quite high, as it would be for multikilowatt beams, the heat may fracture a glass lens or melt polycarbonate. If the radiant energy is concentrated in a small diameter spot, then enhanced heat transfer may result in damage to the surrounding matrix material. The latter may occur for radiant power much less than a kilowatt.
It is even likely possible for powerful lasers that the filter material, glass, or plastic, may be damaged in a time period that is shorter than the time base used to determine the MPE. This is particularly true as the radiant exposure increases. Guidance on typical laser-induced damage threshold levels may be found in ANSI Z136.7 (latest revision). For polycarbonate, these values are 10 J/cm2 (exposure [t] < 10−3 seconds) and 300 t0.5 J/cm2 (exposure [t] ≥ 10−3 seconds). For glass, the values are 1 J/cm2 (exposure [t] < 10−6 second) and 1000 t0.5 J/cm2 (exposure [t] ≥ 10−6 second).
Users of laser-protective eyewear shall be trained to understand potential early signs of damage. These may include, but are not limited to, smoke, flame, incandescence, and luminescence.
Partial Attenuation (Aka Alignment Eyewear)
Alignment Eyewear
Many users confess they take off their eyewear because they cannot see the beam. Alignment eyewear provides a solution to this problem. For alignment of visible beams, conditions may arise that require the user to see the beam through their protective eyewear (cases where remote viewing is not possible). In these situations, the use of alignment eyewear can be approved by the LSO. Alignment eyewear is assigned an OD lower than that which would provide full protection from a direct accidental exposure. For continuous wave lasers, the alignment OD shall reduce irradiance to between Class 2 and Class 3R level. For pulse lasers, the alignment OD shall be no less than the full protection OD – 1.4.
The purpose of alignment eyewear is to allow the user visualization of the beam while lowering the intensity of any beam that is transmitted through the user’s eye-wear to a Class 2 level. To address this issue there is an existing European Norm that recommends OD for alignment eyewear versus the output of lasers used.
Scale Number | 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: (1) administrative liability acknowledgment and acceptance of same, (2) acknowledgment of potential hazards with the utilization of eyewear that does not protect one against small source intrabeam or specularly reflected exposures and finally, (3) collaborative agreement between the LSO and researcher(s) 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 forward.
The trouble with EN207 is on the pulse side, it does not cover today’s laser pulses of nano-, pico-, and femtoseconds. My experience has been a decrease of 1.4 OD is the maximum for alignment eyewear used for pulsed lasers.
Low-Level Adverse Visual Effects
At exposure levels below the MPE, several adverse visual effects from visible laser exposure may occur. The degree of each visual effect is strongest at night and may not be disturbing in daylight. These visual effects are as follows:
Afterimage. A reverse contrast, shadow image left in the visual field after a direct exposure to a bright light, such as a photographic flash. Afterimages may persist for several minutes, depending upon the level of adaptation of the eye (i.e., the ambient lighting).
Flashblindness. A temporary visual interference effect that persists after the source of illumination has been removed. This is similar to the effect produced by a photographic flash and can occur at exposure levels below those that cause eye injury. In other words, flashblindness is a severe afterimage.
Glare. A reduction or total loss of visibility in the central field of vision, such as that produced by an intense light from oncoming headlights or from a momentary laser pointer exposure. These visual effects last only as long as the light is actually present. Visible laser light can produce glare and can interfere with vision even at exposure levels well below those that produce eye injury.
Dazzle. A temporary loss of vision or a temporary reduction in visual acuity.
Startle. Refers to an interruption of a critical task due to the unexpected appearance of a bright light, such as a laser beam.
Surprise Factors That Affect Laser Eyewear Coatings
A common question is “Can I stand in front of any beam?”; the simple answer is you should never stand directly in the path of the beam and the other answer is NO. We are now in the realm of laser filter damage threshold (maximum irradiance) (Figure 16.3). At very high-beam irradiances, filter materials that absorb or reflect the laser radiation can be damaged. It, therefore, becomes necessary to consider a damage threshold for the filter. Typical damage thresholds from Q-switched, pulsed laser radiation fall between 10 and 100 J/cm−2 for absorbing glass, and from 1 to 100 J/cm−2 for plastics and dielectric coatings. Irradiances from continuous wave (CW) lasers, which would cause filter damage, are in excess of those that would present a serious fire hazard, and therefore, need not be considered. Personnel should not be permitted in the area of such lasers. A few words about international standards, EN stands for European Norm, in U.S. terms a standard (Table 16.2).
Figure 16.3
Humid versus dry conditions.
EN 207
Laser eye protection products require direct hit testing and labeling of eye protectors with protection levels, such as D 10,600 L5 (where L5 reflects a power density of 100 MegaWatt/m2 as the damage threshold of the filter and frame during a 10-second direct hit test at 10,600 nm). Filter and frame must both fulfill the same requirements. It is not acceptable to select glasses according to OD alone. The safety glasses must be able to withstand a direct hit from the laser for which they have been selected for at least 10 seconds (CW) or 100 pulses (pulsed mode).
EN 208
This norm refers to glasses for laser alignment. They will reduce the actual incident power to the power of a Class 2 laser (<1 mW for CW lasers). Lasers denoted as Class 2 are regarded as eye safe if the blink reflex is working normally. Alignment glasses allow the user to see the beam spot while aligning the laser. This is only possible for visible lasers (according to this norm visible lasers are defined as being from 400 to 700 nm). Alignment glasses must also withstand a direct hit from the laser for which they have been selected, for at least 10 seconds (CW) or 100 pulses (pulsed mode).
Table 16.2
Scale Numbers
EN 60825
Requires that laser safety eyewear provide sufficient OD to reduce the power of a given laser to equal to or less than the listed MPE levels. It allows specification according to ODs in extreme situations, but recommends the use of EN 207 with a third party laser test. In neither standard is a nominal hazard zone allowed; the only consideration is protection against the worst-case situation such as direct laser radiation.