A laser is a device which produces coherent electromagnetic radiant energy within the optical spectrum from the extreme ultraviolet to the far infrared (submillimetre). The term laser is actually an acronym for light amplification by stimulated emission of radiation. Although the laser process was theoretically predicted by Albert Einstein in 1916, the first successful laser was not demonstrated until 1960. In recent years lasers have found their way from the research laboratory to the industrial, medical and office setting as well as construction sites and even households. In many applications, such as videodisk players and optical fibre communication systems, the laser’s radiant energy output is enclosed, the user faces no health risk, and the presence of a laser embedded in the product may not be obvious to the user. However, in some medical, industrial or research applications, the laser’s emitted radiant energy is accessible and may pose a potential hazard to the eye and skin.
Because the laser process (sometimes referred to as “lasing”) can produce a highly collimated beam of optical radiation (i.e., ultraviolet, visible or infrared radiant energy), a laser can pose a hazard at a considerable distance—quite unlike most hazards encountered in the workplace. Perhaps it is this characteristic more than anything else that has led to special concerns expressed by workers and by occupational health and safety experts. Nevertheless, lasers can be used safely when appropriate hazard controls are applied. Standards for the safe use of lasers exist worldwide, and most are “harmonized” with each other (ANSI 1993; IEC 1993). All of the standards make use of a hazard classification system, which groups laser products into one of four broad hazard classes according to the laser’s output power or energy and its ability to cause harm. Safety measures are then applied commensurate to the hazard classification (Cleuet and Mayer 1980; Duchene, Lakey and Repacholi 1991).
Lasers operate at discrete wavelengths, and although most lasers are monochromatic (emitting one wavelength, or single colour), it is not uncommon for a laser to emit several discrete wavelengths. For example, the argon laser emits several different lines within the near ultraviolet and visible spectrum, but is generally designed to emit only one green line (wavelength) at 514.5 nm and/or a blue line at 488 nm. When considering potential health hazards, it is always crucial to establish the output wavelength(s).
All lasers have three fundamental building blocks:
- an active medium (a solid, liquid or gas) that defines the possible emission wavelengths
- an energy source (e.g., electric current, pump lamp or chemical reaction)
- a resonant cavity with output coupler (generally two mirrors).
Most practical laser systems outside of the research laboratory also have a beam delivery system, such as an optical fibre or articulated arm with mirrors to direct the beam to a work station, and focusing lenses to concentrate the beam on a material to be welded, etc. In a laser, identical atoms or molecules are brought to an excited state by energy delivered from the pump lamp. When the atoms or molecules are in an excited state, a photon (“particle” of light energy) can stimulate an excited atom or molecule to emit a second photon of the same energy (wavelength) travelling in phase (coherent) and in the same direction as the stimulating photon. Thus light amplification by a factor of two has taken place. This same process repeated in a cascade causes a light beam to develop that reflects back and forth between the mirrors of the resonant cavity. Since one of the mirrors is partially transparent, some light energy leaves the resonant cavity forming the emitted laser beam. Although in practice, the two parallel mirrors are often curved to produce a more stable resonant condition, the basic principle holds for all lasers.
Although several thousand different laser lines (i.e., discrete laser wavelengths characteristic of different active media) have been demonstrated in the physics laboratory, only 20 or so have been developed commercially to the point where they are routinely applied in everyday technology. Laser safety guidelines and standards have been developed and published which basically cover all wavelengths of the optical spectrum in order to allow for currently known laser lines and future lasers.
Laser Hazard Classification
Current laser safety standards throughout the world follow the practice of categorizing all laser products into hazard classes. Generally, the scheme follows a grouping of four broad hazard classes, 1 through 4. Class 1 lasers cannot emit potentially hazardous laser radiation and pose no health hazard. Classes 2 through 4 pose an increasing hazard to the eye and skin. The classification system is useful since safety measures are prescribed for each class of laser. More stringent safety measures are required for the highest classes.
Class 1 is considered an “eye-safe”, no-risk grouping. Most lasers that are totally enclosed (for example, laser compact disc recorders) are Class 1. No safety measures are required for a Class 1 laser.
Class 2 refers to visible lasers that emit a very low power that would not be hazardous even if the entire beam power entered the human eye and was focused on the retina. The eye’s natural aversion response to viewing very bright light sources protects the eye against retinal injury if the energy entering the eye is insufficient to damage the retina within the aversion response. The aversion response is composed of the blink reflex (approximately 0.16–0.18 second) and a rotation of the eye and movement of the head when exposed to such bright light. Current safety standards conservatively define the aversion response as lasting 0.25 second. Thus, Class 2 lasers have an output power of 1 milliwatt (mW) or less that corresponds to the permissible exposure limit for 0.25 second. Examples of Class 2 lasers are laser pointers and some alignment lasers.
Some safety standards also incorporate a subcategory of Class 2, referred to as “Class 2A”. Class 2A lasers are not hazardous to stare into for up to 1,000 s (16.7 min). Most laser scanners used in point-of-sales (super-market checkout) and inventory scanners are Class 2A.
Class 3 lasers pose a hazard to the eye, since the aversion response is insufficiently fast to limit retinal exposure to a momentarily safe level, and damage to other structures of the eye (e.g., cornea and lens) could also take place. Skin hazards normally do not exist for incidental exposure. Examples of Class 3 lasers are many research lasers and military laser rangefinders.
A special subcategory of Class 3 is termed “Class 3A” (with the remaining Class 3 lasers termed “Class 3B”). Class 3A lasers are those with an output power between one and five times the accessible emission limits (AEL) for the Class 1 or Class 2, but with an output irradiance not exceeding the relevant occupational exposure limit for the lower class. Examples are many laser alignment and surveying instruments.
Class 4 lasers may pose a potential fire hazard, a significant skin hazard or a diffuse-reflection hazard. Virtually all surgical lasers and material processing lasers used for welding and cutting are Class 4 if not enclosed. All lasers with an average power output exceeding 0.5 W are Class 4. If a higher power Class 3 or Class 4 is totally enclosed so that hazardous radiant energy is not accessible, the total laser system could be Class 1. The more hazardous laser inside the enclosure is termed an embedded laser.
Occupational Exposure Limits
The International Commission on Non-Ionizing Radiation Protection (ICNIRP 1995) has published guidelines for human exposure limits for laser radiation that are periodically updated. Representative exposure limits (ELs) are provided in table 1 for several typical lasers. Virtually all laser beams exceed permissible exposure limits. Thus, in actual practice, the exposure limits are not routinely used to determine safety measures. Instead, the laser classification scheme—which is based upon the ELs applied under realistic conditions—is really applied to this end.
Table 1. Exposure limits for typical lasers
|
Type of laser |
Principal wavelength(s) |
Exposure limit |
|
Argon fluoride |
193 nm |
3.0 mJ/cm2 over 8 h |
|
Xenon chloride |
308 nm |
40 mJ/cm2 over 8 h |
|
Argon ion |
488, 514.5 nm |
3.2 mW/cm2 for 0.1 s |
|
Copper vapour |
510, 578 nm |
2.5 mW/cm2 for 0.25 s |
|
Helium-neon |
632.8 nm |
1.8 mW/cm2 for 10 s |
|
Gold vapour |
628 nm |
1.0 mW/cm2 for 10 s |
|
Krypton ion |
568, 647 nm |
1.0 mW/cm2 for 10 s |
|
Neodymium-YAG |
1,064 nm |
5.0 μJ/cm2 for 1 ns to 50 μs |
|
Carbon dioxide |
10–6 μm |
100 mW/cm2 for 10 s |
|
Carbon monoxide |
≈5 μm |
to 8 h, limited area |
All standards/guidelines have MPE’s at other wavelengths and exposure durations.
Note: To convert MPE’s in mW/cm2 to mJ/cm2, multiply by exposure time t in seconds. For example, the He-Ne or Argon MPE at 0.1 s is 0.32 mJ/cm2.
Source: ANSI Standard Z-136.1(1993); ACGIH TLVs (1995) and Duchene, Lakey and Repacholi (1991).
Laser Safety Standards
Many nations have published laser safety standards, and most are harmonized with the international standard of the International Electrotechnical Commission (IEC). IEC Standard 825-1 (1993) applies to manufacturers; however, it also provides some limited safety guidance for users. The laser hazard classification described above must be labelled on all commercial laser products. A warning label appropriate to the class should appear on all products of Classes 2 through 4.
Safety Measures
The laser safety classification system greatly facilitates the determination of appropriate safety measures. Laser safety standards and codes of practice routinely require the use of increasingly more restrictive control measures for each higher classification.
In practice, it is always more desirable to totally enclose the laser and beam path so that no potentially hazardous laser radiation is accessible. In other words, if only Class 1 laser products are employed in the workplace, safe use is assured. However, in many situations, this is simply not practical, and worker training in safe use and hazard control measures is required.
Other than the obvious rule—not to point a laser at a person’s eyes—there are no control measures required for a Class 2 laser product. For lasers of higher classes, safety measures are clearly required.
If total enclosure of a Class 3 or 4 laser is not feasible, the use of beam enclosures (e.g., tubes), baffles and optical covers can virtually eliminate the risk of hazardous ocular exposure in most cases.
When enclosures are not feasible for Class 3 and 4 lasers, a laser controlled area with controlled entry should be established, and the use of laser eye protectors is generally mandated within the nominal hazard zone (NHZ) of the laser beam. Although in most research laboratories where collimated laser beams are used, the NHZ encompasses the entire controlled laboratory area, for focused beam applications, the NHZ may be surprisingly limited and not encompass the entire room.
To assure against misuse and possible dangerous actions on the part of unauthorized laser users, the key control found on all commercially manufactured laser products should be utilized.
The key should be secured when the laser is not in use, if people can gain access to the laser.
Special precautions are required during laser alignment and initial set-up, since the potential for serious eye injury is very great then. Laser workers must be trained in safe practices prior to laser set-up and alignment.
Laser-protective eyewear was developed after occupational exposure limits had been established, and specifications were drawn up to provide the optical densities (or ODs, a logarithmic measure of the attenuation factor) that would be needed as a function of wavelength and exposure duration for specific lasers. Although specific standards for laser eye protection exist in Europe, further guidelines are provided in the United States by the American National Standards Institute under the designations ANSI Z136.1 and ANSI Z136.3.
Training
When investigating laser accidents in both laboratory and industrial situations, a common element emerges: lack of adequate training. Laser safety training should be both appropriate and sufficient for the laser operations around which each employee will work. Training should be specific to the type of laser and the task to which the worker is assigned.
Medical Surveillance
Requirements for medical surveillance of laser workers vary from country to country in accordance with local occupational medicine regulations. At one time, when lasers were confined to the research laboratory and little was known about their biological effects, it was quite typical that each laser worker was periodically given a thorough general ophthalmological examination with fundus (retinal) photography to monitor the status of the eye. However, by the early 1970s, this practice was questioned, since the clinical findings were almost always negative, and it became clear that such exams could identify only acute injury which was subjectively detectable. This led the WHO task group on lasers, meeting in Don Leaghreigh, Ireland, in 1975, to recommend against such involved surveillance programmes and to emphasize testing of visual function. Since that time, most national occupational health groups have continuously reduced medical examination requirements. Today, complete ophthalmological examinations are universally required only in the event of a laser eye injury or suspected overexposure, and pre-placement visual screening is generally required. Additional examinations may be required in some countries.
Laser Measurements
Unlike some workplace hazards, there is generally no need to perform measurements for workplace monitoring of hazardous levels of laser radiation. Because of the highly confined beam dimensions of most laser beams, the likelihood of changing beam paths and the difficulty and expense of laser radiometers, current safety standards emphasize control measures based upon hazard class and not workplace measurement (monitoring). Measurements must be performed by the manufacturer to assure compliance with laser safety standards and proper hazard classification. Indeed, one of the original justifications for laser hazard classification related to the great difficulty of performing proper measurements for hazard evaluation.
Conclusions
Although the laser is relatively new to the workplace, it is rapidly becoming ubiquitous, as are programmes concerned with laser safety. The keys to the safe use of lasers are first to enclose the laser radiant energy if at all possible, but if not possible, to set up adequate control measures and to train all personnel working with lasers.