ocular exposure will exceed the applicable MPE. The protective factor of the safety filter is normally expressed as a logarithmic quantity referred to as the optical density (D(λ), or just “OD”) for a given wavelength λ. The OD is the negative log10 of the transmittance. Hence a protective factor of one million corresponds to an OD = 6 since the log10 of 1 000 000 = 6.0. Therefore, the required OD for a given exposure is the calculated radiant exposure in J/cm2 divided by the MPE in J/cm2.
There are many commercial laser eye protectors available and standards have been written to test them. There has long been a controversy as to whether some of the tests related to filter damage are realistic and are not just a costly added expense. Some existing testing standards require the eye protection to withstand levels far in excess of skin injury thresholds, with the result that many are critical of these requirements (Figure 6). Efforts in the ANSI Z136 Committee to prepare a more realistic standard for eyewear (ANSI Z136.7) culminated in 2008 with the publication of a new standard for eye protectors and barriers, ANSI Z136.7:2008 and now in a 2nd Edition (2019) (21).
The ANSI Z136.7 standard represents an important advancement in providing manufacturers and testing laboratories with up‐to‐date and realistic test criteria for laser‐protective spectacles and goggles. Just what is important to specify and test has long been a focus for spirited discussions in laser‐safety committees. The first European standards were based upon German standards from the 1970s that attempted to provide users with specifications for laser eyewear that already incorporated MPE limits. The German standards committee had a strong background in testing and labeling of other forms of personal protective equipment and placed an early emphasis on testing for failure. One criticism of this early standard was that the user's needs appeared to play a secondary role, and the marking was not very informative unless one really knew the marking code (22, 23).
FIGURE 6 The absurdity of testing laser eye protectors to withstand kilowatt laser beams.
The ANSI standards subcommittee, SSC‐7, that prepared ANSI Z136.7 was particularly interested in developing standardized test methods for newer types of eye‐protectors. Many newer designs employ reflective technologies, such as holographic reflectors, since the true level of protection of such multi‐wavelength designs depend upon pupil diameter, eye movements, a fit tolerance, and eye relief as well as simply the optical properties of the filter. These require very involved point‐by‐point testing over the surface for these sophisticated filter systems.
To minimize the cost and complexity of filter‐material damage thresholds, the ANSI Z136.7 standard introduced the concept of default damage (failure) threshold values for substrate materials, where – unless the manufacturer wanted to claim substantially higher laser‐induced damage thresholds – damage testing would not be required. Values for plastics, glass, and quartz were specified, and the manufacturer could refer to these time‐dependent damage‐threshold values without the need for testing each product. As an example, the failure threshold value (TVmax) for polycarbonate lenses is given as 10 J cm−2 for pulse durations t < 1 ms and 300 t0.5 J cm−2 for t > 1 ms. Glass and quartz have higher values of TVmax for CW lasers, but actually somewhat lower TVmax for short pulse durations. Of course, testing for saturable absorption (transient, reversible bleaching) of absorbers from very‐short‐pulse lasers would always be required, and radiant exposure test levels for Q‐switched and mode‐locked laser pulses are provided. The ANSI TSC‐7 subcommittee members felt that it was inappropriate to place much emphasis on substrate damage of goggle materials at levels well above skin injury thresholds, and hopefully this issue, which has been around for decades, will soon be forgotten (11, 24, 25).
8 LASER BARRIERS
Laser barriers are defined in the ANSI Z136.7 standard as “moveable or fixed equipment (devices) used to block or attenuate laser energy,” include curtains or shields that are used for enclosing a “laser controlled area” (see Figure 6). Laser curtains are particularly useful for nonpermanent laser operational situations; for example, where they are put up at an entryway to enclose a “temporary laser controlled area” during servicing. The ANSI Z136.7 standard places a number of obviously important requirements on the materials used in laser barriers, e.g. the material should not be flammable or emit toxic by‐products when irradiated. The manufacturer should specify the material damage threshold for a given time (traditionally 60 or 100 seconds, but now standardized at 100 seconds). This “barrier threshold limit” (TL), or damage threshold, for a fabric curtain ranges between approximately 5 and 350 W cm−2. For metal barriers, the TL will typically be of the order of approximately 1200 W cm−2 for three minutes. Active barriers, such as those with built‐in sensors to shut down laser operation before the barrier fails, have TL values with very high irradiances (Figure 7).
The ANSI Z136.7 standard is not just for the manufacturer of protective eyewear and barriers. The LSO is tasked with the potential need to calculate the separation distance, Ds, of the laser from the barrier if the maximum laser beam irradiance exceeds the TL of the barrier. Typically, in large industrial material processing applications the beam is focused and the beam irradiance decreases rapidly beyond the beam's focal point, and this calculation becomes both useful and necessary.
Manufacturer testing protocols for the TL are provided to determine the highest irradiance incident on a laser barrier for which no penetration occurs for an exposure of 100 seconds and at a specified exposure diameter ranging between 3 and 10 mm. In addition, the test should report the “first visible damage,” or FVD, such as a visually observable change or structural alteration in the protective barriers surface (melting, pitting, cracking, discoloration, and so on) that occurs during or following the exposure; and any flame, smoke or a sign of thermal distortion should be recorded and airborne contaminants must be captured and analyzed for toxic content. A penetration threshold level (PTL) is also to be recorded; this is the initial power level at which beam breakthrough of the material occurs.
The ANSI Z136.7 standard also lists requirements for the manufacturer to provide certain information to the user of a barrier, such as any limitations of use, such as certain applications. A description of an eye‐protective filter optical density with the angular protection afforded by a reflective filter, if applicable, must accompany eyewear. In addition, both groups of products must be appropriately labeled and directions for storage, care, cleaning, and periodic inspections, including any chemical‐exposure warnings are to be provided.
FIGURE 7 Laser barrier curtains used to enclose a “controlled area” where large equipment must move in and out, at entryways or for establishing a “temporary laser controlled area” during servicing.
All laser‐protective barriers must contain labels with certain minimal information, such as manufacturer and the TL and exposure time for which that limit applies and the exposure conditions under which protection is specified.
9 CONCLUSIONS
Laser technology has become mature in the past three decades and is ubiquitous throughout industry, science, and medicine. Laser safety programs are encountered in a large variety of workplaces. The keys to the safe use of lasers are firstly: enclose the laser radiant energy if possible; and secondly, if not possible, control measures become essential where training of those working with lasers becomes paramount for safe use.
Bibliography