As Laser Use Increases, So Does the Importance of Safety
May 15, 2001
Addressing laser beam hazards, safety regulations for laser use, and methods to improve laser safety should be foremost in the mind of every laser machine operator.
In today's industrial workplace, lasers are used for cutting and welding metals, bar code scanning, inventory tracking, parts identification, measurement, as well as rapid prototyping. Simply put, the uses are many, and the applications seem limitless.
In a recent report issued by the National Research Council on "Harnessing Light," the authors state, "As we move into the next century, light will play an even more significant role." In the world of industrial processing, where lasers are becoming an increasing presence, two significant questions must be asked: Are these new high-power devices being used safely, and what can be done to ensure their safe use at my facility?
Understanding the different types of lasers and the potential hazards that they can produce is important.
High-powered neodymium-YAG (Nd:YAG) and carbon dioxide (CO2) lasers are used to cut or weld materials with a high degree of accuracy. These high-powered lasers require greater safety measures than some of the lower-power vision system lasers used for inventory tracking or parts identification.
In today's industrial environment, laser uses include cutting, alignment, annealing, drilling, dynamic balancing, metrology, nondestructive testing, sealing, soldering, and lithography—to mention just a few.
The types of laser technology, the wavelength at which they operate, and the applications in which they are used.
These applications are done generally with a broad range of laser types, including gas, solid-state, metal vapor, diode, and dye. The active medium is different in each laser to achieve different beam characteristics (such as the wavelength) that are useful in different applications. These are summarized in Figure 1.
Skin Hazards. Because the eye is much more susceptible to damage from laser radiation than other parts of the body, skin hazards have not been emphasized as much. However, repeated, or even a single, exposure to certain laser wavelengths can cause skin damage of varying degrees.
Furthermore, the proliferation of high-power laser systems, particularly the newer excimer laser operating in the ultraviolet spectral region, has increased the possibility that personnel may be exposed to potentially hazardous radiation levels.
Eye Hazards. Eye hazards can be significant when some laser types are used. The first, and perhaps the most important, factor in determining a laser's eye hazard potential is its wavelength. Wavelength determines which part of the eye absorbs the radiation and whether the radiation can be focused by the eye.
Eye injuries are caused by thermal or photochemical mechanisms that occur when a laser beam interacts with the eye. If the beam enters the eye (possible with visible and near-infrared lasers), the beam energy is concentrated by the lens of the eye about 100,000 times at the retina. Thus, even a small amount of laser light can cause eye damage. An individual exposed to these beams could not discern the large "E" on the Snellen chart in an eye doctor's office, which means that visual acuity is reduced to 10-200 or worse.
Exposure to a laser beam may occur in several ways (see Figure 2). Direct exposure occurs when one is in the direct path of the laser beam or in the path of a beam reflecting off a mirror-like object.
Indirect exposure occurs when a beam is scattered before it reaches the eye or skin. The material scattering the laser energy may be a rough, nonreflective surface, such as a brick wall, or it may be small, airborne particles, such as dust or water vapor. During indirect exposure, the beam's energy dissipates rapidly as one moves away from the material that caused the scatter.
A special case of direct viewing exists when a person is in the path of a scanning laser beam. Products such as bar code readers typically use a laser beam to scan at high speeds. Because the beam crosses the eye rapidly, the time of exposure is very brief, and the hazard is normally less than that of a stationary beam of equal power. Although the hazard potential is less if the beam is scanning, a serious potential for injury still exists with a high laser power.
Nonbeam Hazards. Laser safety issues also include nonbeam hazards. During safety audits of industrial and research laser facilities, the following types of basic safety problems are found repeatedly:
1. Laser-produced fires
2. Toxic fume production
3. Unprotected wiring and tubing
4. Water, dye, and chemical spills
5. Unposted or improper warning signs
6. Improper fume exhaust systems
7. Defeated interlocks
8. No lockout/tagout provisions
9. Lack of data on toxicity of chemicals and fumes (No material safety data sheets [MSDS] information)
All of these factors must be addressed in an overall laser safety analysis of a high-power laser facility.
In the United States, the major government agencies concerned with laser safety regulations are:
The Center for Devices and Radiological Health (CDRH), a regulatory bureau within the Federal Food and Drug Administration (FDA) of the Department of Health and Human Services. CDRH was chartered by Congress to standardize the performance safety of manufactured laser products. All laser products manufactured and entered into commerce after August 2, 1976, must comply with these regulations.
The CDRH regulation is known as the Federal Laser Product Performance Standard (FLPPS) and is identified as 21CFR subchapter parts 1040.10 and 1040.11.
Once a manufacturer files a CDRH certification report, it may actively market a laser. This requires the manufacturer to classify the laser into one of four main hazard classes. To verify that a laser has been classified and certified by the manufacturer, the federal government requires that a certification label be affixed to all lasers. If this label cannot be found, the laser may not have been manufactured according to federal standards.
The American National Standards Institute (ANSI), an organization in which expert volunteers participate on committees to set industry consensus standards in various fields. ANSI has been the basis for existing federal standards as well as for the Suggested State Regulations for Lasers (SSRL) legislation. ANSI has established a family of major laser safety standards. The principal standard for industry is ANSI Z136.1, which provides requirements and recommendations for the safe use of lasers in typical industrial and research environments.
The Occupational Safety and Health Administration (OSHA), the user-based regulatory federal government agency within the Department of Labor that is involved with laser safety.
Currently, OSHA does not have a comprehensive laser standard. Instead, OSHA's practice has been to rely on accepted industry practice, such as those specified in the ANSI Z136.1 standard, in addition to requiring compliance with the FDA/CDRH laser standard. Although OSHA does not have a comprehensive laser standard, it does publish several specific laser-standard and guideline documents.
In addition, Alaska, Arizona, Arkansas, Florida, Georgia, Illinois, Maine, Montana, New York, Pennsylvania, Texas, and Washington have laser standards that generally impose user requirements and/or registration. In addition, several professional societies including the Laser Institute of America (LIA)and the American Conference of Governmental Industrial Hygienists (ACGIH)provide laser safety assistance.
In some applications where open beams are required (industrial processing, laser robotics, etc.), areas of potentially hazardous exposure must be defined.
This is done by determining the nominal hazard zone (NHZ), defined as the space within which the level of direct, reflected, or scattered radiation exceeds the level of the applicable maximum permissible exposure (MPE). Persons exposed outside the NHZ boundary are exposed below the MPE level and are considered to be in a safe location.
The purpose of an NHZ evaluation is to define where control measures are required. Thus, the classic method of controlling lasers by enclosing them in an interlocked room has become limiting and, in many instances, can be an expensive over reaction to the actual hazards present.
If the NHZ evaluation produces NHZ values that are small compared to the dimensions of the laser unit and, more specifically, small compared to the area around the laser unit where workers are present during operation, the required control measures can be significantly reduced. For example, the ANSI Z136.1 [(1993): Section 220.127.116.11] states: "Frequently the hazard analysis will define an extremely limited NHZ and procedural controls can provide adequate protection."
Most industrial lasers fall into the higher classifications of Class 3B and Class 4 (meaning they are potential eye, skin, and even fire hazards) unless they are totally enclosed and interlocked and there is no beam access during normal system operation.
In today's robotic environments, the beam path of many lasers is confined by design to limit access significantly. In other areas, the beam path is totally open. In each case, the controls required vary as follows:
Totally Enclosed Beam Path. Perhaps the most common form of a Class 1 laser system is a high-power laser inside a protective enclosure that is equipped with appropriate interlocks and/or labels on all removable panels or access doors. The beam is not accessible during operation and maintenance.
When properly labeled and safeguarded with protective housing interlocks (and all other applicable engineering controls), this system fulfills all requirements under the ANSI Z136.1 user's standard for a Class 1 laser and may be operated in the enclosed manner with no additional safeguards for the operator.
It should be noted that during service or maintenance, controls appropriate to the class of the embedded laser may be required (perhaps on a temporary basis) when the beam enclosures are removed and beam access is possible. Beam access during maintenance or service procedures does not alter the Class 1 status of the laser during operation.
Limited Open Beam Path. A common work practice, particularly with industrial lasers, is to have an enclosure that surrounds the area around the laser focusing optics and encloses the immediate area of the workstations almost completely. For example, a computer-controlled positioning table may be located within the enclosure. The design allows for a gap of less than 0.25 inch between the bottom of the enclosure and the top of the material to be laser-processed. This design provides the needed mobility for the part to be laser-cut or welded, for example, relative to a stationary laser delivery optic.
This system design might not meet the stringent human-access requirements of the federal government's laser standard for a Class 1 laser. Nonetheless, this design provides a limited open beam path. In this situation, the ANSI Z136.1 standard recommends that the laser safety officer (LSO) perform a laser hazard analysis and establish the extent of the NHZ.
In many system designs, the NHZ is extremely limited, and procedural controls (rather than elaborate engineering controls) are sufficient to ensure safe use. In many cases, the laser units may be reclassified by the site LSO as Class 1 under the specifications of the ANSI Z136.1 standard.
This installation requires a detailed standard operating procedure (SOP). Training commensurate with the class of the embedded laser is also needed for the system operator. For example, protective equipment (eye protection, temporary barriers, clothing, etc.) is needed only if the hazard analysis indicates a need or if the SOP requires periods of beam access during setup or infrequent maintenance activities.
Totally Unenclosed Beam Path. Several specific applications exist in which high-power (Class 3B and Class 4) lasers are used with an unenclosed beam. This includes open industrial processing systems (often incorporating robotic delivery) and laser research laboratory installations. These laser uses require a hazard analysis and NHZ assessment by the LSO. Controls are chosen reflecting the magnitude of hazards associated with the accessible beam.
When the entire beam path from a Class 3B or Class 4 laser is not sufficiently enclosed and/or baffled, creating the possibility of exposure to radiation above the MPE, a laser controlled area is required. During periods of service, a temporary controlled area may be established. The controlled area encompasses the NHZ. The major controls that are required are summarized as follows:
1. Entryway Controls. Specific controls are required at the entryway to a laser controlled area. These can be summarized as follows:
a. All personnel entering a laser area shall be adequately trained and provided proper laser protective eyewear.
b. All personnel shall follow all applicable administrative and procedural controls.
c. All area/entryway controls shall allow both rapid entrance and exit under all conditions.
d. The controlled area shall have a clearly marked panic button (disconnection switch) that allows rapid deactivation of the laser.
In addition, the ANSI Z136.1 standard provides options that allow the LSO to provide an entryway control suited to a Class 4 installation. The options include using entryway interlocks and/or procedural entryway controls, often in conjunction with entryway warning systems.
2. Temporary Controlled Area. Should overriding interlocks become necessary to gain access to Class 3B or Class 4 lasers during periods of special training, service, or maintenance, a temporary laser controlled area must be devised following specific procedures approved by the LSO.
These procedures must outline all necessary safety requirements during such operation. Temporary laser controlled areas, which by their nature do not have the built-in protective features of a laser controlled area, provide all of the safety requirements for personnel, within and outside of the temporary laser controlled area during periods of operation when the interlocks are defeated. Temporary protective measures for service are handled in a manner similar to service of any embedded Class 4 laser.
3. Engineering Controls. Engineering controls are features designed into the laser equipment or facilities to minimize the risk of exposure to hazardous beams. The most common engineering controls are protective housings and enclosures that cover the equipment or the beam path. Interlocks are often placed on the protective housings so that if they are removed, the beam is shut off. Beam stops that provide safe termination of the beam's path are also effective engineering controls.
Labels, such as those discussed in the previous section, and signs that give notice of lasers operating in a given area are also valuable safety controls.
4. Protective Equipment. Protective equipment such as protective barriers or curtains, protective clothing, or protective eyewear should be relied upon only if the other control measures do not provide adequate protection.
5. Laser Barriers and Protective Curtains. Area control can be accomplished in some cases using special barriers or curtains. Similar in appearance to welding curtains, the barriers are designed to withstand either direct or diffusely scattered laser beams. Barriers and curtains must be opaque to the laser wavelength, cannot be combustible, and must be designed to withstand the intensity of the laser beam that they are exposed to.
6. Protective Clothing. Where personnel may be exposed to levels of radiation that clearly exceed the MPE for the skin, protective clothing must be used. Ultraviolet lasers and Class 4 lasers pose the most serious threat of skin injury. Protective clothing for use with Class 4 lasers should be fire-resistant and should not melt. For diffuse reflections of UV energy, tightly woven clothing provides adequate protection.
7. Protective Eyewear and Window Filters. Filters used for eye protection are typically made of absorbent glass or plastic. The transmission data of each filter is usually available from the eyewear supplier. Laser protective eyewear filters are specified in terms of the optical density at specific wavelengths. The optical density of a filter specified at a given laser wavelength is determined by a logarithmic equation.
Personal protective equipment, including protective eyewear, must be used whenever Class 3B or Class 4 lasers are operating. Protective eyewear is usually not required for operators of Class 2 or Class 3A lasers, unless a person's viewing position is very close to the laser beam, because a person's aversion response to bright light will cause the eye to blink and the head to turn away in case of an accidental exposure. If a person is forced to view a bright light for a prolonged time, laser protective eyewear may be needed even for Class 2 or Class 3A lasers.
As might be expected, the more dangerous Class 3B and Class 4 lasers have the greatest number of controls associated with their use. Because of the greater level of danger presented by these lasers, they must be operated in restricted areas, and personnel within those areas must wear protective eyewear.
Operators of Class 3B and Class 4 lasers must have extensive laser safety training. Personnel using Class 1 laser equipment do not need specific laser safety training unless their work requires the removal of portions of the protective housing, which would expose a hazardous beam inside the equipment.
A person who is not trained or authorized to enter a posted Class 3B or Class 4 laser area should not violate any of the warnings given at the door. These may include signs, flashing lights, or a combination of these. Overriding the entryway warnings should be allowed only in cases of emergency inside the room.
8. Administrative and Procedural Controls. Administrative and procedural controls consist of a series of rules, regulations, and SOPs that are designed to minimize the risk of exposure. One of the most effective administrative controls is training. People simply work more safely if they understand the potential hazards. Reducing the number of unnecessary spectators in a laser work area is also important in minimizing the potential risk of exposure.
The key person in any laser safety control program is the LSO. The LSO is to "have authority and responsibility to monitor and enforce the control of laser hazards and to effect the knowledgeable evaluation and control of laser hazards" (ANSI Z136.1). The LSO is reponsible for ensuring that effective control measures have been established and the controls are being used.
Actions recommended for Class 3B, and often required by the LSO for Class 4, lasers are as follows:
1. Ensure direct supervision by an individual knowledgeable in laser safety.
2. Require approved entry of any noninvolved personnel.
3. Terminate all potentially hazardous beams in a beam stop of an appropriate material.
4. Use diffusely reflecting materials near the beam, when appropriate.
5. Provide personnel within the laser controlled area with appropriate laser protective eyewear.
6. Secure and locate lasers so that the beam paths are above or below eye level in any standing or seated position.
7. Have all windows, doorways, open portals, etc., in an indoor facility covered or restricted to reduce transmitted beams below appropriate ocular MPE level.
8. Require storage or disabling of lasers when not in use.
Several good work practices must be observed when working around lasers. These include the following:
1. Never intentionally look directly into a laser.
2. Do not stare at the light from any laser. Allow yourself to blink if the light is too bright.
3. Do not view a Class 3A (or any higher-power) laser with optical instruments.
4. Never direct a beam toward other people.
5. Position the laser so that it is well above or below eye level.
6. Store lasers that are not in use to prevent unauthorized use by untrained personnel.
Today, the field of lasers continues to develop. Manufacturers are learning how to obtain greater power levels from smaller units. Laser uses expand as new applications are announced each month.
In its infancy, laser technology was a little-understood and complex process, used only in experimental laboratories by a few scientists. Today, the laser has become a commonly used tool for industry and will be for many decades to come. For that growth to continue, safety practices must be followed, especially as higher-power, smaller devices become more common.