4.0  MODE OF OPERATION

Lasers can operate in one of the following three modes: 1) continuous wave, 2) pulsed, or 3) q-switching.

Continuous Wave (CW)

Continuous wave lasers produce a steady stream of photons. Energy is pumped into the system at a rate that equals the light output. Beam characteristics are easily measured because the laser has reached a steady state condition. While most CW lasers use a gas medium, they can be constructed in a wide variety of lasing materials. An extensive array of wavelengths may be produced. The He-Ne laser was the first CW laser. Power levels can range from a few milliwatts for He-Ne lasers to several kilowatts for carbon dioxide lasers.

Extremely high power levels can be obtained by using a technique known as "q-switching" that momentarily stores excess energy. A shutter is placed in the optical path to prevent laser emission until a very large population inversion has built up. When the shutter is opened the electrons rapidly fall to the ground state releasing a tremendous pulse of energy that lasts only a few nanoseconds. Powers in the megawatt range can be produced by this technique. Q-switching is commonly used with ruby and neodymium solid lasers.

5.0  CHARACTERISTICS OF LASERS

The light from a laser differs from conventional light in four basic ways: 1) intensity, 2) coherence, 3) collimation, and 4) monochromaticity.

Intensity

Laser light contains a high concentration of energy per unit area of the beam. Lasers that emit only a few milliwatts of power can produce a highly intense beam 1-2 millimeters in diameter that will not diverge over a very long distance. Although lasers produce highly intense light, only a few types of lasers are truly powerful because intensity is defined as power per unit area. By comparison, an ordinary light bulb is more powerful than a typical laser, but the light is not collimated and consequently spreads out. For example, the light irradiance from a 1 mW He-Ne laser can be 6 orders of magnitude greater than that from a 100 W incandescent lamp at a distance of approximately 20 meters. Some lasers can produce thousands of watts continuously while others can produce trillions of watts in a nanosecond pulse.

Coherency

Ordinary light is incoherent or out of phase; light waves start at different times and move in all directions. Laser light, however, is coherent because it is the result of stimulated emission. All the waves produced by the laser are lined up or in phase with each other. The crests and troughs of each wave line up exactly and reinforce each other. The new light wave starts out exactly in phase with the photon that stimulated it. Holograms are based on this property of laser light.

Collimation

Because laser light is coherent it is highly collimated or directional. Laser beams are narrow, travel in virtually parallel lines, and will not spread out or diverge as light from most normal sources. Because of this small divergence the intensity of laser light, unlike ordinary light, is fairly constant over long distances. This property of lasers significantly increasing the hazard potential of the beam. The beam from a typical inexpensive laser will spread only about one meter after traveling one kilometer. Because laser beams are highly collimated they act as if they come from a distant point source. The beam can be easily focused to a small point by a simple lens, dramatically increasing the energy concentration of the beam. The property of collimation allows laser light to accurately measure distances, aim rifles, and track flying targets.

Reflections, however, reduce the collimation of the laser beam and result in beam divergence. Laser beams are reflected to some extent from all surfaces. If the reflecting surface is mirror-like, the reflection is termed specular. The reflection is called diffuse if the reflecting surface is rough. The spreading is greater when the reflection is from a rough or diffuse spreading surface. Beam intensity can be reduced by a factor of hundreds to thousands. The roughness of the diffuse reflector is more important than the color in reducing intensities.

Monochromaticity

Unlike ordinary light, which is composed of all the colors of the spectrum, laser light is composed of only one color. All the light waves in the beam are composed of the same wavelength. Each laser produces its own characteristic color of light.

Some lasers are tunable and can be adjusted to produce several different colors, but they can emit only one color at a time. Because laser light is monochromatic it can be used to selectively interact with materials, revealing information about the structure of atoms and molecules. Laser light is approximately 10 million times more monochromatic than conventional light sources.

6.0  BIOLOGICAL EFFECTS

The eye is extremely vulnerable to injury if exposed to the beams from most types of lasers. The type of injury depends upon the intensity of light, its wavelength, and the tissue exposed. Damage results from either temperature or photochemical effects. Acute exposure may result in corneal or retinal burns. Cataract formation or damage to the retina may result from chronic exposure to laser light. Retinal damage is of particular concern from exposure to wavelengths in the visible and near infrared region.

Most sources of incoherent light can be viewed safely because the light reaching the eye is only a small portion of the total output and the energy is spread over the entire retina. Laser radiation, however, is composed of coherent light. The beam can pass through the pupil and focus on a very small spot on the retina, depositing all its energy on this area. Only visible and near infrared radiation is focused on the retina. Damage to the retina may result in limited or total blindness if the optic nerve is injured. Injury may be irreversible and there may be no pain or discomfort from the exposure.

In addition to heat damage, some very high-powered, short-pulsed lasers such as the carbon dioxide and the Nd YAG mode-locked laser can cause shock waves that can mechanically disrupt the retina and cause the eye to hemorrhage.

Damage to the skin is also possible from exposure to laser beams. Acute exposure may cause injuries ranging from mild reddening to blistering and charring. Skin cancers may result from chronic exposure to ultraviolet light. The extent and type of damage depends on the amount of energy deposited and the wavelength of the light. Unlike injury to the eye, acute damage to the skin is usually repairable.

Ultraviolet radiation (200-400 nanometers)

Exposure to the eye from ultraviolet light in the 200 nm to 315 nm range is absorbed by the cornea and may cause photokeratitis (corneal inflammation). Unlike the skin, repeated exposure of the cornea to ultraviolet light does not result in a protective mechanism. Near ultraviolet light between 315 nm and 400 nm is absorbed largely in the lens and may cause cataracts. Wavelengths less than 400 nm do not pose a hazard to the retina.

Exposure to the skin from lasers that emit in the UV region may cause a photochemical reaction resulting in reddening, aging, and possibly skin cancer. Examples of lasers operating in the ultraviolet region include the neodymium:YAG-Quadrupled (QSW & CW), and the Ruby (doubled) laser.

Visible and Near-Infrared Radiation (400 to 1400 nanometers)

Exposure to laser beams in the visible (400 nm - 700 nm) and near-infrared (700 nm - 1400 nm) regions of the spectrum may damage the retina. Laser beams in this region are readily transmitted by the eye and focused by the lens to produce an intense concentration of light energy on the retina. The incident exposure on the cornea can be concentrated by a factor of approximately 100,000 times at the retina due to this focusing effect. This energy is converted to heat and may cause a retinal burn resulting in visual loss or even blindness if the optic nerve is injured. Even low energy laser beams, if concentrated by a factor of 100,000, can cause damage to the eye. For this reason wavelengths in the 400 nm to 1400 nm range are termed the ocular hazard region.

Exposure to the skin from laser beams in the visible and infrared regions may cause photosensitive reactions, skin burns and excessive dry skin. Lasers operating in the visible region of the spectrum include the ruby, neodymium:YAG (doubled), helium-cadmium, helium-neon, argon, and krypton. Lasers operating in the near-infrared region include the neodymium:YAG, gallium arsenide, and helium-neon.

Middle and Far-Infrared Radiation (1,400 -10,000 nanometers)

Laser beams in the middle and far-infrared regions produce injury primarily to the cornea and to a lesser extent the lens. Damage is usually from heating effects, although pulsed lasers such as the carbon dioxide laser may cause injury from thermomechanical effects. Virtually no light reaches the retina beyond 1400 nm. Middle-infrared radiation between 1,400 nm and 3,000 nm may penetrate deep into the lens causing cataracts. Far-infrared radiation in the 3000 nm to 10,000 nm range is absorbed by the cornea and may cause corneal burns and loss of vision.

The major danger to the skin from lasers operating in this region is burn damage. Lasers operating in this region include: hydrogen fluoride, carbon monoxide, carbon dioxide, and hydrogen cyanide.

7.0  LASER SAFETY STANDARDS

Laser safety standards are derived from government mandated regulations and voluntary standards. Safety rules governing the manufacture of lasers are established by the Federal Government. Laser products manufactured after August 2, 1976 must conform to performance standards established by the Food and Drug Administration (FDA), (21 CFR 1040.10). The standard requires that lasers be properly classified and labeled by the manufacturer. Thus, for most lasers, measurements or calculations to determine the hazard classification are not necessary. In addition, the standard establishes certain engineering requirements for each class and requires warning labels that state maximum output power. Although the performance standard regulates the manufacture of lasers it does not address the safe use of lasers.

OSHA Regulations (29 CFR 1926.54 and 102) specify generalized rules for the safe use of lasers in the construction industry. These include user training, posting and labeling requirements, laser safety goggles, and maximum exposure intensities. Requirements for the use of laser goggles in general industry are specified in sections 1910.132-133. The safe use of lasers in general industry (including research laboratories) is covered under OSHAs General Duty Clause which states that employers must furnish employees a place of employment that is free from recognized hazards that are likely to cause death or serious injury.

Extensive recommendations for the safe use of lasers have been developed by the American National Standards Institute (ANSI) and adopted by the American Conference for Governmental Industrial Hygienists (ACGIH). Lasers are classified according to their relative hazards, and appropriate control measures for each class are specified. Compliance with these standards generally eliminates the need for measurement of laser radiation, quantitative analysis of hazard potential, or use of the Threshold Limit Value (TLV). Compliance with ANSI and OSHAs Construction Standard will usually satisfy the requirements of the OSHA General Duty Clause.

8.0  LASER CLASSIFICATION

Since August, 1976 manufacturer's have been required by Federal law to classify lasers. If the class is not known, it can be determined by measurements and/or calculations. Lasers are classified according to the ability of the primary or reflected beam to injure the eye or skin. The appropriate class is determined from the wavelength, power output, and duration of pulse (if pulsed). Classification is based on the maximum accessible output power. There are four laser classes, with Class 1 representing the least hazardous. All lasers, except Class 1, must be labeled with the appropriate hazard classification.

Class 1

Class 1 laser devices cannot produce damaging radiation levels to the eye even if viewed accidentally. Prolonged staring at the laser beam however, should be avoided as a matter of good practice. This class has a power output less than 0.4 uW for CW lasers operating in the visible range. A completely enclosed laser of a higher classification is categorized as a Class 1 laser if emissions from the enclosure cannot exceed Class 1 limits. If the enclosure is removed, e.g. during repair, control measures for the class of laser contained within are required.

Class 2

Class 2 lasers are incapable of causing eye injury within the duration of the blink, or aversion response (0.25 sec). Although these lasers cannot cause eye injury under normal circumstances, they can produce injury if viewed directly for extended periods of time. Class 2 lasers only operate in the visible range (400 nm to 700 nm) and have power outputs between 0.4 uW and 1 mW for CW lasers. The majority of Class 2 lasers are helium-neon devices.

Class 3a

Class 3a lasers cannot damage the eye within the duration of the blink or aversion response. However, injury is possible if the beam is viewed through binoculars or similar optical devices, or by staring at the direct beam. Power outputs for CW lasers operating in the visible range are between 1 mW and 5 mW.

Class 3b

Class 3b lasers can produce accidental injuries to the eye from viewing the direct beam or a specularly reflected beam. Class 3b laser power outputs are between 5 mW and 500 mW for CW lasers. Except for higher power Class 3b lasers, this class will not produce a hazardous diffuse reflection unless viewed through an optical instrument.

Class 4

Class 4 lasers are the most hazardous lasers. Exposure to the primary beam, specular reflections, and diffuse reflections are potentially hazardous to the skin and eyes. In addition, class 4 lasers can ignite flammable targets, create hazardous airborne contaminants and usually contain a potentially lethal high voltage supply. The power output for CW lasers operating in all wavelength ranges is greater than 500 mW. All pulsed lasers operating in the ocular focus region (400 nm to 1400 nm) should be considered Class 4.

Unknown Class

Laser classification can be determined by measuring the output irradiance or radiant exposure using instruments traceable to the National Bureau of Standards. These measurements should only be performed by qualified personnel. The laser class can also be determined from calculations. For CW lasers, the wavelength and average power output must be known. Classification of pulsed lasers requires the following information: wavelength, total energy per pulse (or peak power), pulse duration, pulse repetition frequency (PRF), and emergent beam radiant exposure. In addition to the above information laser source radiance and maximum viewing angle subtended by the laser must be known for extended-source lasers, such as injection laser diodes. Detailed information on classifying lasers may be found in the ANSI Guide for the Safe Use of Lasers.

9.0  CONTROL MEASURES FOR LASER RADIATION

The recommended procedure for using lasers safely is to identify the class involved and then comply with the appropriate control measures. Control measures are designed to reduce the possibility of exposure to the eye and skin from hazardous laser radiation. Preventing ocular exposure from the primary beam and specular reflections is of primary concern because serious injury to the unprotected eye is possible.

To control potential hazards, priority should be given to the use of engineering controls. Although commercial laser products manufactured after August 2, 1976 incorporate many engineering controls, the use of additional controls should be considered to reduce the risks.The most effective engineering control is to totally enclose the laser and all beam paths.

The potential for exposure to hazardous laser radiation is probably greatest in the research laboratory where open and high power laser beams are frequently encountered. Because researchers need flexibility to change optical arrangements and make adjustments during experimental procedures, laser beams cannot always be totally enclosed. Generally in research laboratories where engineering controls cannot be relied upon, administrative controls, protective eyewear, and properly supervised laser controlled areas must be rigorously enforced to reduce hazards.

Because OSHA standards are generally non-specific, users are encouraged to adopt the following requirements and recommendations for the safe use of lasers. These practices are based on the FDA Performance Standard, OSHA Regulations, and the ANSI Hazard Classification System. Engineering controls specified by the FDA Performance Standard apply only to manufacturers of laser equipment but should be incorporated by users when practical.

Protective Housing (All Classes-FDA)- A protective housing shall be provided by the manufacturer to prevent access to laser radiation which exceeds the intended classification. If the protective housing is removed, as in research laboratories, control measures shall be based on the maximum accessible radiation.

Safety Interlock (All Classes-FDA)- Any portion of the protective housing designed to be removed without tools shall be interlocked to prevent access to radiation in excess of the applicable class.

Optical Viewing Devices and Windows (All Classes)- Viewing optics (including lenses, telescopes, microscopes, and viewing windows) shall not be used to view the direct beam or specular reflections unless an appropriate filter is used or adequate laser protective eyewear is worn to reduce laser radiation to safe levels.

Maximum Exposure (All Classes)- Workers shall not be exposed to light intensities above:

• Direct Staring: 1 microwatt per square centimeter
• Incidental Observing: 1 milliwatt per square centimeter
• Diffused Reflected Light: 2.5 watts per square centimeter

Directing beam at people (All Classes)- The laser beam shall not be directed at people or into potentially occupied areas. Experiments should be designed so that the laser beam is not at eye level.

Registration (Classes 3a, 3b, 4)- These lasers shall be registered with the Radiation Safety Office.

Reduction of Intensities ( Class 2, 3a, 3b, 4)- If full output power is not needed, the laser beam intensity should be reduced to a less hazardous level by using absorbing filters or beam shutters.

Alignment (Class 2, 3a, 3b, 4)- Alignment of laser optical components (mirrors, lenses, beam deflectors, etc.) shall be done in a manner that ensures the eyes are not exposed to hazardous levels of radiation.

Firm Laser Mount (Class 2, 3a, 3b, 4)- The laser should be mounted on a firm support to ensure that the beam travels along its intended path.

Unattended Operation (Class 2, 3a, 3b, 4)- Operating lasers should not be left unattended for appreciable lengths of time. Lasers should be turned off, or beam shutters or caps should be used when laser transmission is not required.

Emission Indicator (Class 2, 3a, 3b, 4-FDA)- Laser systems shall incorporate an emission indicator that provides a visible or audible signal during radiation emission.

Safety Eyewear (Class 2, 3a, 3b, 4)- Laser eyewear designed to protect for the specific wavelengths of the laser shall be worn when exposure to hazardous levels of laser radiation are possible. Laser safety eyewear shall be worn in areas where unenclosed Class 3b or 4 lasers are operated. The optical density of the eyewear shall reduce the laser radiation to the eye to safe levels. Safety eyewear shall be labeled with the optical density of the lens and the wavelength that it protects against.

Beam Stop or Attenuator (Class 3a, 3b, 4)- Potentially hazardous beams should be terminated by a permanently attached beam stop or attenuator. The beam stop should be non-reflecting and fire-retardant.

Training (Class 3a, 3b, 4)- Personnel shall receive training in basic laser safety procedures. Training should include the potential hazards associated with the use of the laser and control measures necessary to minimize exposure to laser radiation. Training should be an on-going program, both for new employees and experienced personnel, and when new equipment is used. Written standard operating procedures, and a training manual should be presented to each individual assigned to work with a Class 3 or 4 laser. Documentation of training should be readily available.

Medical Surveillance (Class 3b, 4)- Personnel operating Class 3b and 4 lasers should receive appropriate eye and skin examinations prior to laser use. Examinations should be repeated every three years.

Remote Interlock Connector (Class 3b, 4-FDA)- A remote interlock connector shall be provided to allow electrical connections to an emergency disconnect interlock or a door interlock.

Spectators (Class 3b, 4)- Spectators should not be allowed in a laser controlled area containing an unenclosed Class 3b or 4 laser unless permission has been obtained from the supervisor, potential hazards have been explained, and appropriate protective measures taken.

Beam shutters (Class 3b, 4)- Lasers should be equipped with a beam shutter that covers the aperture. The shutter should completely stop the laser beam. A black, non-reflective material is desirable.

Warning System (Class 3b, 4)- A visible or audible warning device that activates prior to laser emission should be provided. The delay time should be sufficient to allow personnel to avoid exposure to the beam.

Key-Switch (Class 3b, 4-FDA)- Class 3b and 4 lasers shall be provided with a keyed switching device. The key shall be removable and the laser shall be inoperable once the key has been removed.

Enclosed Beam Path (Class 3b, 4)- The entire beam path, including the target area, should be enclosed if possible. Safety interlocks should be used with the enclosure. If the beam is enclosed the laser could revert to a less hazardous laser classification.

Laser Controlled Area (Class 3b)- Unenclosed Class 3b laser systems should be operated in a controlled area.

1. The laser controlled area should be under the direct supervision of an individual knowledgeable in laser equipment and trained in laser safety procedures.

2. Spectators should be prevented from entering the laser area during the operation of the laser.

3. The area shall be posted with appropriate laser warning signs.

4. Specular surfaces should be removed from the beam path, if possible. If removal is not possible the surfaces should be painted a flat-black color or covered with a diffuse material. The intended target should be a diffuse, absorbing material to prevent reflections.

5. To reduce the possibility of specular reflections personnel should not wear watches or jewelry in controlled areas.

6. Appropriate eye protection shall be worn by all personnel potentially exposed to hazardous levels of radiation.

Laser Controlled Area (Class 4)- Unenclosed Class 4 lasers shall be restricted to a well controlled area suitably administered and designed to protect personnel from exposure to laser radiation. The following requirements are in addition to those for a Class 3b controlled area:

1. Access to the controlled area shall be restricted to authorized personnel during the operation of the laser.

2. Doors to the laser controlled area shall be provided with safety interlocks or a guard must be posted to prevent unexpected entry during laser operation.

3. A warning light connected to the power supply or shutter must be installed on doors leading to the laser facility.

4. A "panic switch" should be readily available for deactivating the laser during an emergency.

5. The work area shall be separated from the surrounding environment by walls, panels, or black flameproof heavy curtains.

6. The laser controlled area must be posted with the appropriate warning sign and a notice that protective eyewear must be worn before entering the restricted area.

7. The facility should be light tight. Windows and other openings should be covered to prevent the transmission of hazardous laser radiation to potentially occupied areas.

8. If possible, Class 4 lasers should be operated by remote control and monitored by television.

9. Walls in the laser laboratory should be dark, dull, and non- reflecting.

10.0  LABELS

Warning signs and labels shall be in accordance with the FDA Performance Standard and ANSI Standards. Signs and labels shall be conspicuously displayed on equipment and on access doors where applicable. Additional precautionary instructions, such as eye protection required, should be included on the sign or label.

Class 1:  Warning labels and signs are not required for Class 1 lasers. Enclosed Class I lasers containing more hazardous laser radiation within shall have a warning label located on the access panel.

Class 2:  The label must include the laser hazard symbol and the words "CAUTION- Laser Radiation- Do Not Stare into Beam", the class, and type of laser. Warning signs on doors are not required.

Class 3a:  Signs and labels shall include the laser hazard symbol and bear the words "CAUTION- Laser Radiation- Do Not Stare into Beam or View Directly with Optical Instruments ", the class, and type of laser. Entrances to laser areas should be posted with a warning sign.

Class 3b:  Signs and labels shall include the laser hazard symbol and bear the words "DANGER- Laser Radiation- Avoid Direct Exposure to Beam", the class, and type of laser. Doors leading to the laser area shall be posted with warning signs.

Class 4:  Appropriate warning signs must be posted on equipment and doors leading to the facility. Signs and labels shall include the laser hazard symbol and bear the words "Danger- Laser Radiation- Avoid Eye or Skin Exposure to Direct or Scattered Radiation", the class, and type of laser. Additional precautions or protective actions should be provided as needed.

11.0  INVISIBLE RADIATION

Because infrared and ultraviolet radiation are invisible, special precautions must be used when working with lasers that emit radiations in these regions. Labels and warning signs should specify that lasers produce invisible radiation. In addition to the above requirements, the following recommendations should be used when working with infrared or ultraviolet lasers:

Infrared:  For Class 3 & 4 lasers, the beam path should be terminated with a highly absorbent, non-reflecting backstop. Caution should be used because metal surfaces that appear dull can cause specular reflections of infrared radiations. In addition, for Class 4 lasers, the beam path should be terminated by a fire resistant backstop such as firebrick. The backstop should be inspected periodically for degradation. The beam and target area should be shielded with infrared absorbing materials, such as lucite or plexiglass, to minimize reflections from Class 3b or Class 4 lasers.

Ultraviolet:  Exposure to ultraviolet radiation should be minimized by using materials that attenuate the radiation to safe levels. Special attention should be given to the possibility of producing undesirable reactions in the presence of ultraviolet radiation, e.g. ozone.

12.0  ASSOCIATED HAZARDS

In addition to the hazards of the laser beam, protection is also necessary from other hazards associated with the operation of the laser. These hazards include explosions, electrical shocks, cryogenic liquids, flammable liquids, noise, x-rays, UV radiation, and airborne contaminants.

Explosion Hazards

Lasers and ancillary equipment may present explosion hazards. High pressure arc lamps and filament lamps used to excite the lasing medium should be enclosed in housings that can withstand an explosion if the lamp disintegrates. In addition, the laser target and elements of the optical train may shatter during laser operation and should be enclosed in a suitable protective housing. Capacitors may explode if subjected to voltages higher than their rating and should be adequately contained. High energy capacitors should be enclosed in one-eighth inch thick steel cabinets.

Electrical Hazards

Lethal electrical hazards may be present, especially around high power laser systems. Continuous wave lasers use direct current or radiofrequency power supplies and pulsed lasers employ large capacitor banks for electrical storage. Lasers and associated electrical equipment should be designed, constructed, installed and maintained in accordance with the American National Standard Electrical Code. Electrical circuits greater than 42.5 volts are considered hazardous unless limited to less than 0.5 mA.

To reduce electrical hazards high voltage sources and terminals should be enclosed. Power should be turned off and all high-voltage points grounded before working on power supplies. Operators should not stand on metal floors, or in water while working with live electronic equipment. Accessible, non-current carrying metallic parts of laser equipment must be grounded. Electrical circuits should be evaluated with respect to fire hazards.

Cryogenics

Cryogenic liquids (especially liquid nitrogen) may be used to cool the laser crystal and associated receiving and transmitting equipment. These liquified gases are capable of producing skin burns and may replace the oxygen in small unventilated rooms. The storage and handling of cryogenic liquids should be performed in a safe manner. Insulated handling gloves of quick removal type should be worn. Clothing should have no pockets or cuffs to catch spilled cryogenics. Suitable eye protection should be worn. If a spill occurs on the skin, the area should be flooded with large quantities of water. Adequate ventilation should be present in areas where cryogenic liquids are used.

Flammable Hazards

Flammable solvents, gases, and combustible materials may be ignited by a Class 4 laser beam. Laser beams should be terminated by a non-combustible material such as a brick. Combustible solvents or materials should be stored in proper containers, and shielded from the laser beam or electrical sparks. Lasers and laser facilities should be constructed and operated to eliminate or reduce any fire hazard. Unnecessary combustible materials should be removed in order to minimize fire hazards. Laser laboratories should contain an appropriate fire extinguisher.

Noise

Noise levels in laser laboratories can exceed safe limits because of high voltage capacitor discharges. Hearing protection may be required.

X-Rays

X-ray production is possible when high voltages exceed 15 kV. Although most laser systems use voltages less than 8 kV, some research models may operate above 20 kV. Laser systems capable of producing ionizing radiation must be surveyed by the Safety Office to ensure that x-ray levels are within legal limits.

Ultraviolet Radiation

Although laser radiation presents the chief hazard, it may not be the only optical hazard. Laser discharge tubes and pumping tubes may emit hazardous levels of ultraviolet radiation and should be suitably shielded. Particular care should be used with quartz tubes. Most lasers now use heat-resistant glass discharge tubes which are opaque in the UV-B (280 nm-315 nm) and UV-C (100 nm-280 nm) spectrum.

Chemical Hazards

Vaporized target materials, toxic gases, vapors and fumes may be present in a laser area. Ozone is produced around flash lamps and concentrations of ozone can build up with high repetition rate lasers. Asbestos fibers may be released from the firebrick used as backstops for carbon dioxide lasers.

The increasing use of chemical lasers may introduce chemical hazards that are more dangerous than the laser radiation. For example, fluorides used in fluoride lasers are highly toxic and demand immediate emergency measures upon contact. He-Cd lasers may contaminate the laboratory with toxic cadmium vapors if the exhaust gases are not vented to the outside. The dyes that are the active medium of tunable lasers are often very toxic and may cause acute or chronic skin problems. Some dyes may be carcinogenic. Gloves, laboratory coats, and proper eye protection must be worn when handling hazardous chemicals. An eyewash station and emergency shower shall be available in areas where there is a possibility that hazardous chemicals may be splashed in the eyes or on the skin. Food or drink should not be consumed in the laser lab if potential toxic air contaminants or chemicals are present. Adequate general ventilation or local exhaust must be maintained in laser installations to ensure that toxic air contaminants are below acceptable limits.

13.0  THRESHOLD LIMIT VALUE

The Threshold Limit Value (TLV) is the amount of laser radiation that nearly all individuals may be exposed to without experiencing adverse effects. TLVs have been developed for exposure to the eye and skin. TLVs for the eye are based on possible exposure to the direct beam or from extended diffuse reflections. No person shall operate a laser in a manner that exposes the unprotected eye or skin to laser radiation in excess of the applicable TLV. If engineering controls are not suitable to reduce levels below the TLV then laser safety eyewear must be worn.

If the laser classification is known it is generally unnecessary to perform calculations or monitor the laser environment to determine if exposure of the eyes or skin is dangerous. Appropriate control measures for the classification should be implemented. For example, if a class 3b or 4 laser is used, the TLV for the eyes may be exceeded from viewing the direct or specular beam and appropriate eye protection must be worn. Because only Class 4 lasers can produce hazardous diffuse reflections, calculations for viewing diffuse reflections from class 1, 2, and 3 systems are not necessary.

The TLV should be determined if the laser has not been classified by the manufacturer and a quantitative evaluation of the hazard associated with a laser is needed. Monitoring is often improperly performed and has a high degree of error. Calculations should be used in most situations to determine if exposures are below the applicable TLV. The ANSI Guide for the Safe Use of Lasers should be consulted to calculate TLVs and the latest edition of the ACGIH Threshold Limit Values should be consulted for current TLVs (available from the Safety Office). Irradiance and radiant exposure are used to describe exposure limits from direct laser sources. Exposure limits from extended sources are described in radiance and time-integrated radiance.

14.0  LASER SAFETY EYEWEAR

The energy emitted from lasers is highly concentrated and can cause permanent eye injury. Although engineering controls are preferred to reduce hazards from the laser beam, it may be necessary to use laser safety eyewear when engineering controls are inadequate. The eyewear should be matched to the wavelength emitted and provide proper attenuation for the laser intensity. Laser safety eyewear must be clearly marked with the optical density of the lens and wavelengths for which protection is provided. The following factors should be considered when purchasing laser protective eyewear:

Wavelength:  The wavelength of the laser output must be known. If the laser emits more than one wavelength, each wavelength must be considered.

Optical Density:  The attenuation of laser light by protective goggles is given by its optical density (OD). The OD must be sufficient to reduce the laser light to safe levels while transmitting sufficient ambient light for safe visibility. The OD is measured on a logarithmic scale, thus a filter that attenuates a beam by a factor of 1,000 has an OD of 3 and a filter with an OD of 6 attenuates the beam by a factor of 1,000,000. The required OD is determined from the maximum intensity that an individual could be exposed.

Laser Beam Intensity:  The maximum irradiance in watts/square centimeter for CW lasers and the maximum radiant exposure in joules/square centimeter for pulsed lasers must be known.

Luminous Transmittance:  When considering laser safety goggles the visible or luminous transmission must be considered along with the optical density. Luminous transmission is given in percent transmission of visible light. Laser goggles with a lower luminous transmittance than required may result in eye fatigue and accidents. However, proper optical density should not be sacrificed for increased luminous transmission.

Damage Threshold: The resistance of the lenses to damage from the laser beam must be considered. The damage can take the form of bubbling, melting, or shattering. The lens must be capable of absorbing the amount of energy under the most severe operating conditions without suffering changes in the light transmission characteristics. Laser goggles should be inspected periodically for pitting, cracking, discoloration and deterioration in the mountings.

Comfort:  Laser safety eyewear should be comfortable and provide a good fit. Spectacle type frames generally are more comfortable than goggles but do not fit as tight and may allow unattenuated laser radiation to reach the eyes. However, the increased comfort of spectacle type frames may increase user acceptance.

Lenses: Lenses in laser safety eyewear can be made of reflective glass or plastic, absorptive glass filters, or absorptive polymeric filters. Reflective lenses can create potentially hazardous specular reflections. These lenses are also heavy and uncomfortable and can scratch easily, allowing potentially hazardous radiation to reach the eye. Absorptive filters are not affected by surface scratches nor do they create potentially hazardous reflections. Polymeric filters, such as polycarbonate, offer several advantages over glass absorptive filters. They are lighter, have better impact resistance, and can withstand higher energy densities. Lenses should be mounted in goggle type frames to ensure maximum protection. No lens material is useful for all wavelengths and for all radiant exposures.

15.0  MEDICAL SURVEILLANCE 

Personnel who routinely use Class 3b or Class 4 lasers should be enrolled in the Medical Surveillance Program for examination of the eyes and skin. The examination should include a medical history, with emphasis on the eyes and skin. Current and past medication usage should be reviewed, particularly the use of photosensitizing drugs. Visual acuity should be determined and the structures of the eye that could be injured from the wavelengths emitted should be examined. For example, workers using laser systems operating in the wavelength range of 400 to 1400 nm should have retinal examinations. Employees working with ultraviolet lasers or who have a history of photosensitivity should have skin examinations. Workers who only occasionally use Class 3b or 4 lasers should have a visual acuity test.

Medical examinations should be performed prior to employment and following any suspected laser injury. Eye examinations should be repeated every three years to detect injury or subtle changes that may have occurred from chronic exposure.