Although accidents occur, laser systems are designed to be safe. The objective of
safe design is to insure that the equipment controls, interlocks, beam enclosures,
shutters, and filters are appropriate to the hazard potential of the systems and the
experience level of personnel operating and servicing the equipment. The goal of
restricting human access to hazardous levels of optical radiation (or live electric
currents) is usually achieved by permanent interlocks which are designed to be failsafe
or failure-proof. For example, extensive use is made of mechanical-electrical
interlocks. In this instance, a lateral or rotary movement of a hinge or a latch
activates the switch which is in the power circuit for the laser. If the contacts are
activated, the system will not operate. Interlocks are designed to require intentional
operation to inactivate or bypass the interlock. This design of interlocks is to insure
that even partial opening of a panel to a point where hazardous radiation can
be emitted from the opening results in shutdown. Additionally, positive-activated
switches (e.g., “deadman” type) are often used to insure operator alertness and
reduce the risk of accidental firing.
For certain applications laser projections are used. In such instances, it is often
desirable to alter the output beam pattern of a hazardous laser so a relatively safe
pattern results. Methods to accomplish this include the use of wide beams, unfocused
beams or beam diffusers. A CW laser with an emergent beam diameter of 10 -
20 cm is 100-times less hazardous than a laser of the same power with a 2 mm beam
diameter. An unfocused beam is safer because the biological effect depends upon
the total power and the beam irradiance. A diffuser is used to spread the beam over
a greater area and thus change the output from intrabeam viewing to an extended
source. Generally, the actual classification of the laser would not change unless the
output beam diameter were greater than 80 mm. In theory, a diffuser could change
a Class IV laser into a Class I or II laser; however, in practice, diffusers are most economical in reducing the hazard classification approximately one class. The safety
applied to indoor laser installations usually depends upon the class of the laser.
• Class I (exempt) laser systems do not require much control. The user may opt to post the area with a Low Power Laser sign. The laser should be labeled with the beam characteristics. Some Class IIIb or Class IV laser systems are embedded in closed devices (e.g., printers). For such systems, the manufacturer normally installs enclosure interlocks and service panels to prevent tampering and persons using the system must receive training on hazards and controls for that laser before being designated an authorized user.
• Class II (low power) lasers require a few more controls. This is the first instance when posting the area with a CAUTION sign becomes mandatory. Additionally, non-reflective tools are often used to reduce reflected light. Controls applied to the system include blocking the beam at the end of its useful path, controlling spectator access to the beam, and controlling the use of view ports and collecting optics.
• Class IIIa lasers are widespread (e.g., laser pointers, levelers, and gun scopes are Class IIIa) and potentially hazardous when using optics. Thus, posting of the area with either CAUTION or DANGER signs depends upon the irradiance. Personnel maintaining such systems or conducting research with unenclosed beams should be given a baseline eye exam. Other controls which may be necessary to prevent direct beam viewing and to control specular reflections are:
1. Establish alignment procedures that do not include eye exposure
2. Control fiber optic emissions
3. Establish a normal hazard zone for outdoor use
4. Consider eye protection if accidental intra-beam viewing is possible
• Class IIIb laser systems are potentially hazardous if the direct or specularly reflected beam is viewed by the unprotected eye, consequently eye protection may be required if accidental intra-beam viewing is possible. It is at this point that many of the suggested controls become mandatory. Besides posting the area with DANGER signs, other control measures include:
1. Laser operated only by authorized operators who are trained on the systems laser hazards
2. Baseline eye exam required for maintenance and research applications
3. Spectators must be under the direct supervision of the operator
4. Laser power controlled by a key-operated master switch
5. Beam stops mandatory
6. Laser area interlocks (for CW power levels greater than 15 mW)
• Class IV laser systems that are pulsed visible and IR-A lasers are hazardous to the eye for direct beam viewing, and from specular (and sometimes diffuse) reflections. Ultraviolet, infrared, and CW visible lasers present a potential fire and skin hazard. The safety precautions associated with these high-risk lasers generally consist of publishing and adhering to an operational safety procedure manual; using door interlocks to prevent exposure to unauthorized or transient personnel entering the controlled area; the use of baffles to terminate the primary and secondary beams; and the wearing of protective eye wear or clothing by personnel within the interlocked facility.
1. Safety interlocks at the entrance of the laser facility shall be so constructed that unauthorized or transient personnel shall be denied access to the area while the laser is capable of emitting laser radiation at Class IV levels.
2. Laser electronic firing systems for pulsed lasers shall be so designed that accidental pulsing of a stored charge is avoided. Additionally, the firing circuit shall incorporate a fail-safe (e.g., deadman) system.
3. An alarm system including a muted sound and/or warning lights (visible through laser protective eye wear) and a countdown procedure should be used once the capacitor banks begin to charge.
4. Good ambient illumination is essential when eye protection is being worn. Light colored, diffuse surfaces assist in achieving this goal.
5. Operate high-energy/high-power lasers by remote control firing with television monitoring. This eliminates the need for personnel to be physically in the room with the laser. However, enclosing the laser, the laser beam, and the target in a light-tight box is a viable alternative.
6. Because the principal hazard associated with high-power CW far-infrared (e.g., CO2) lasers is fire, a sufficient thickness of earth, firebrick, or other fire-resistant materials should be provided as a backstop for the beam.
7. Reflections of far-infrared laser beams should be attenuated by enclosure of the beam and target area or by eye wear constructed of a material that is opaque to laser wavelengths longer than 3 μm (e.g., Plexiglas). Remember, even dull metal surfaces may be highly specular at far-infrared laser wavelengths (e.g., CO2 - 10.6 μm).
Warning signs and labels are used to alert workers. Placarding of potentially hazardous areas should be accomplished for Class IIIb and IV lasers. Appropriate warning labels shall be affixed permanently to all Class II, III, and IV lasers and laser systems. Class II and IIIa usually use CAUTION signs/labels while class IIIb and IV use DANGER signs/labels.
A laser operational safety procedure manual is a document used to describe both a systems potential hazards and controls implemented to reduce the risk of injury from the laser. It may detail specific administrative controls (e.g., signs, lights), engineering controls (e.g., interlocks, enclosures, grounding, ventilation), required personal protection (e.g., eye wear, clothing) and training (laser safety, chemical safety).
As a minimum, an operational safety procedure must be promulgated for:
• Class IV laser systems.
• Two or more Class III lasers with different operators and no barriers.
• Complex or nonconforming interlock systems or warning devices.
• Modifications of commercial lasers which have decreased safety.
• Class II, III, or IV laser systems used outdoors or off-site.
• Beams of Class II, III, or IV laser which must be viewed directly or withcollecting optics near beam.
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