Playing it Safe

Following standards and using appropriate safety equipment can help eliminate the risk of eye injuries during laser use.

The high radiance or brightness of a laser provides the laser’s great value in material processing and laser surgery, but it also accounts for its significant hazard to the eye. Even a small helium-neon (HeNe) alignment laser is typically ten times as bright as conventional bright-light sources such as a xenon arc or the sun.

The most serious eye injuries typically take place in R&D laboratories. Laboratory staff should alert students and newcomers from the start to the risks of working with lasers. Too often laser safety does not become an issue until someone loses eyesight. Instructing students in laser safety does not have to be a boring presentation of rules, however. Behind the safety standards are many interesting questions in physiological optics, vision, and the biophysics of laser-tissue interactions.

Examining the eye

A collimated beam entering the relaxed human eye will experience an increase of irradiance by about five orders of magnitude. In other words, a beam with irradiance of 1 W/cm2 at the cornea is focused to a spot of 100 kW/cm2 at the retina. The retinal image size is only about 15 to 20 µm, which is considerably smaller than the diameter of a human hair. Users may dismiss the hazard posed by such a small laser burn in their retina, reasoning that their eye contains millions of cone cells.

In reality, retinal injury is always larger because of heat flow and acoustic transients, and even a small disturbance of the retina can damage vision. This is particularly important in the area of central vision, the macula lutea (yellow spot), or simply the macula. The central region of the macula, the fovea centralis, measures about 150 µm in diameter and is responsible for detailed 20/20 vision. Damage to this extremely small central region can result in severe vision loss, even though 95% of the retina is unscathed. The surrounding retina is useful for movement detection and other tasks but possesses little acuity; for example, the reason your eye must move across a line of print in order to read is because your detailed vision covers a very small angular field.

At wavelengths beyond the 400- to 1400-nm retinal hazard region, the cornea—and even the lens—can be damaged by laser beam exposure. Staring into a UV or blue continuous-wave laser can lead to photochemical injury.

Injury by accident

Laser eye injuries can happen in a multitude of ways but always involve a laser beam inadvertently diverted into the eye. For example, a researcher in a physical chemistry laboratory is aligning the output beam of a neodymium-doped yttrium- aluminum-garnet (Nd:YAG) laser-pumped optical parametric oscillator (OPO) to direct it into a gas cell to study photodissociation parameters for a particular molecule. Leaning over a beam director, he glances down over an upward, secondary beam and approximately 80 µJ enters his left eye. The impact produces a microscopic hole in his retina. A small hemorrhage is produced over his central vision, and he sees only red in his left eye. Within an hour he is rushed to an eye clinic where an ophthalmologist tells him he has only 20/400 vision.

In another example, a physics graduate student attempts to realign the internal optics in a Q-switched Nd:YAG laser system—a procedure normally performed by a service representative, but one that the student has witnessed several times before. A «weak» secondary beam reflected upward from a Brewster window enters the student’s eye and produces a similar hemorrhagic retinal with a severe loss of vision.

Though such accidents occur each year, often they do not receive publicity because of administrative reasons or litigation. Because almost all open-beam lasers pose a severe hazard to the eyes, scientists and engineers must wear eye protection or observe other safety measures. However, laser users often make excuses for not wearing eye protection. All too common, the familiar phrases «I know where the beams are,» «I don’t place my eye near a beam,» and «Safety goggles are uncomfortable» are heard in the lab. Unfortunately, in most laser-related accidents, eye protectors were available but not worn.

Injuries do not always happen when eye protectors are not worn because the probability that a small beam will intersect the 3 to 5 mm pupil of a person’s eye is small. For the people who don’t wear goggles, the risk of injury appears to be acceptably low or they would choose protection. But consider the following scenario: If these same individuals were given an air rifle loaded with 100 BBs, placed in a stainless-steel-lined cubical room 4 m on a side, and told to fire all of the BBs in a random direction, how many would be willing to do this without heavy clothing and eye protectors? Not many—the risk of firing without protection would seem too high. Yet the probability of being hit with a BB in that scenario is about the same as that of sustaining a laser eye injury when not wearing goggles.

Comfortable laser goggles exist. The common complaint that one cannot see the beam to align it is readily solved; for example, the image converters and various fluorescent cards used to align IR beams can be used for visible lasers as well.

Nonvisible beams can pose subtle hazards. Our instinctive aversion to bright lights that can trigger a reaction to visible beams does not come into play for IR and UV beams—users may not know to look away from damaging beams until it is too late. The black anodized surface used in many lab setups can be a good reflector in the IR spectral region, which sets the stage for damage by a reflected beam.

Safety standards

Laser safety standards group all lasers into four general hazard classes and provide safety measures for each class. U.S. federal regulations require all commercial laser products to have a label indicating the hazard class. The safety measures recommended in these standards include beam blocks, shields, baffles, and eye protectors.

Documents such as the American National Standard, ANSI Z136.1-2000, «The Safe Use of Lasers,» provide maximum permissible exposure (MPE) limits as sliding scales with wavelength and duration for all wavelengths from 180 nm to 1 mm and for exposure durations of 100 fs to 8 hr. The exposure limits of the American Conference of Governmental Industrial Hygienists (ACGIH) and ANSI became the basis for the U.S. Federal Product Performance Standard (21 CFR 1040) mentioned above.8 With the proliferation of ultrafast lasers used in research laboratories, the MPEs recently have been updated to include limits for these picosecond and femtosecond pulse durations.

The development of exposure limits in the subnanosecond time domain has been difficult because the interaction mechanisms of laser radiation with biological tissues differs. Nonlinear damage mechanisms do not scale in the same way with wavelength, pulse duration, and retinal image size as do thermal and thermal acoustic damage mechanisms. For this reason, it has been necessary to perform a number of studies of damage mechanisms. Likewise, researchers have had to use histological techniques to study the range of effects that occur at subvisible threshold levels. Before setting exposure limits, the standard-setters must understand the consequences of exceeding the threshold.

Eye protection

The ANSI Z136.1 standard for eye protection includes factors such as comfort and fit, filter damage threshold, and periodic inspection, as well as the critical specification of wavelength and optical density. Opinions differ on how to rate eye protection. European standards emphasize filter burn-through and damage thresholds; in U.S. laser safety, officers are reluctant to demand that laser protective filters have damage-resistant irradiance values far exceeding skin damage thresholds.

Ordinary polycarbonate eye protectors can withstand irradiances of about 100 W/cm2 (1 MW/m2) for several seconds of illumination by a 10.6 µm wavelength. Because of this, some have proposed an alternative protocol to damage-based testing that would instead evaluate materials for saturable absorption by testing the optical density (OD) under CW and Q-switched irradiation conditions.

The most pressing issue in laser safety standards now is requiring laser eye protection to be marked in an intelligible fashion to ensure that the user will not misunderstand and select the wrong goggle. Several eye injuries appear to have been caused in the past few years by a user choosing the wrong protector for the wavelength. This is particularly likely in an environment where different laser wavelengths are in use, as in a research laboratory or a dermatological laser operation. In addition to the wavelength, the terms «ruby,» «neodymium,» «carbon-dioxide,» and similar labels should be required on the specification sheet. In a location with a multiwavelength laser, however, this could still lead to confusion.

When used properly, lasers can be useful, productive instruments and tools. If scientists and engineers working with lasers use protective goggles, beam blocks, and other safety equipment regularly, they will significantly reduce the risk of injury to themselves and to others. Instructors have an ethical obligation to teach newcomers the risks of working with lasers and common-sense safety procedures. As laser safety becomes commonplace, catastrophic injury will become a thing of the past.

(By David Sliney SPIE,  OEmagazine,  July, 2001)

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