SECTION 4: LASER CONTROLL MEASURES

• Control Measures by Laser Classification
• Warning Signs and Labels
• Protective Equipment

Individuals who operate lasers should follow the guidelines in this section to protect both themselves and others in the area.  Supervisors and operators should be properly trained before working with or around Class 2, 3, and 4 lasers

Features of a laser device, such as power output, beam diameter, pulse length, wavelength, beam path, beam divergence, and exposure duration determine the capability for injuring personnel.  The potential for injury from use of a laser is determined by its classification, therefore, the control measures are also determined by laser class. 

Concepts such are the maximum permissible exposure (MPE), accessible emission level (AEL) and  nominal hazard zone (NHZ) are important for the laser operator to use and understand.

Maximum Permissible Exposure (MPE) (top)

MPE is the maximum level of laser radiation to which a person may be exposed without hazardous effects or biological changes in the eye or skin.  The MPE is determined by the wavelength of of laser, the energy involved, and the duration of the exposure.  The ANSI 136.1 standard tables 5, 6, and 7 (See Appendix A) summarize the MPE for particular wavelengths and exposure durations.  

MPE is a necessary parameter in determining the appropriate optical density and the nominal hazard zone.

Optical Density (OD) (top)

The OD (absorbance) is used in the determination of the appropriate eye protection.  OD is a logarithmic function defined by:

Where H₀ is the anticipated worst case exposure conditions (in joules/cm² or watts/cm²) and the MPE is expressed in the same units as H₀.  The OD values for various lasers, computed for various appropriate exposure times, are listed below.  Keep in mind that these values are for intrabeam viewing (worst case) only.  Viewing Class 4 diffuse reflections (such as alignment tasks) requires, in general, less OD.  These should be determined for each situation and would be dependent upon the laser parameters and viewing distance.

Table 4 provides a summary of optical density needed for particular lasers, based on the worst case exposure duration.

Table 4. Optical Densities for Protective Eyewear for Various Laser Types

Normal Hazard Zone (NHZ) (top)

The NHZ relates to the space within which the level of direct, reflected, or scattered radiation during normal operation exceeds the appropriate MPE.  Exposure levels beyond the NHZ are below the appropriate MPE level, thus no control measures are needed outside the NHZ.  The NHZ may be calculated using the following formula:

Where f is the emergent beam divergence measured in radians; F is the radiant power (total radiant power for continuous wave lasers or average radiant power of a pulsed laser) measured in watts; and a is the diameter of the emergent laser beam, in centimeters.

Control Measures by Laser Classification (top)

Potential hazards exist to all individuals working near a laser system.  Such individuals should be warned of the existence and location of lasers, and of the meaning of the warning labels for all classes of lasers. 

Particular attention should be given to the environment where the laser is used.  This factor should be considered together with the class and application of the laser for determining the control measures to be applied.  Basic elements to be considered are:

·       number and class of lasers

·       laser location

·       presence (access) of uninformed, unprotected personnel

·       permanence of beam paths

·       presence of objects that may have specular surfaces or reflecting objects near the beam path

·       use of optical devices such as lenses, microscopes, etc.

Control measures may be broken down to two types: administrative controls, such as signage, procedures, etc., and engineering controls, such as beam housings, shutters, etc.  The following are general considerations for work with lasers, per laser hazard class.  Table 5 provides a summary of these control measures.

Class 1 (top)

Many Class 1 lasers have higher class lasers enclosed within a protective housing.  If the Class 1 laser has an enclosed Class 3b or 4 laser, interlocks should be provided on any removable parts of the housing, or the laser should have a service access panel that is either interlocked or requires a tool for removal.  If the protective housing is removed, control measures appropriate for the enclosed laser class should be followed. 

All Class 1 lasers must be labeled.

Class 2 (top)

Class 2 lasers must be labeled. 

The laser beam should not be purposefully directed toward the eye of any person.  Alignment of the laser optical systems (mirrors, lenses, beam deflectors, etc.) should be performed in such a manner that the primary beam, or specular reflection of the primary beam, does not expose the eye to a level above the MPE for direct irradiation of the eye.

The work area should be posted with a warning label or sign cautioning users to avoid staring into the beam or directing the beam toward the eye of individuals.

If the MPE is exceeded, design viewing portals and/or display screens to reduce exposure to acceptable levels.

If the Class 2 laser has an enclosed Class 3b or 4 laser, interlocks should be provided on any removable parts of the housing, or the laser should have a service access panel that is either interlocked or requires a tool for removal.  If the protective housing is removed, control measures appropriate for the enclosed laser class should be followed. 

Class 3a (top)

Class 3a lasers must be labeled accordingly.  The work area should be posted with a warning label or sign cautioning users to avoid staring into the beam or directing the beam toward the eye of individuals.

Removable parts of the housing and service access panels should have interlocks to prevent accidental exposure.  A permanent beam stop or attenuator may also be used.

If the MPE is exceeded, design viewing portals and/or display screens to reduce exposure to acceptable levels.  Alignment procedures should be designed to ensure the MPE is not exceeded.

Class 3b (top)

Class 3b lasers and laser systems must be labeled accordingly.  These lasers are used in areas where entry by unauthorized individuals can be controlled.  If an individual who has not been trained in laser safety must enter the area, the laser operator or supervisor should first instruct the individual as to safety requirements and must provide protective eyewear, if required.

If the entire beam is not enclosed or if a limited open beam exists, the laser operator, supervisor or laser safety officer should determine a Nominal Hazard Zone (NHZ).  An alarm, warning light or verbal countdown should be used during use or start up of the laser.

The controlled area should

·       have limited access to spectators,

·       have beam stops to terminate potentially dangerous laser beams,

·       be designed to reduce diffuse and specular reflections,

·       have eye protection for all personnel,

·       not have a laser beam at eye level,

·       have restrictions on windows and doorways to reduce exposure to levels below the MPE, and

·       require storage or disabling of the laser when it is not being used.

If the MPE is exceeded, design viewing portals and/or display screens to reduce exposure to acceptable levels.  Alignment procedures and collecting optics should be designed to ensure the MPE is not exceeded.

Only authorized, trained individuals should service the laser.  Approved, written standard operating, maintenance and service procedures should be developed and followed.

Class 4 (top)

In addition to the control measures described for Class 3b, Class 4 lasers should be operated by trained individuals in areas dedicated to their use.  Failsafe interlocks should be used to prevent unexpected entry into the controlled area, and access should be limited by the laser operator to persons who have been instructed as to the safety procedures and who are wearing proper laser protection eyewear when the laser is capable of emission.

Laser operators are responsible for providing information and safety protection to untrained personnel who may enter the laser controlled areas as visitors.

The laser area should be

·       restricted to authorized personnel only

·       designed to allow for rapid emergency egress

·       equipped with a device that allows for deactivation of the laser or reduction of the output to below the MPE

·       designed to fulfill Class 3b controlled area requirements

·       designed with entry safe controls

·       designed such that the laser may be monitored and fired from a remote location

·       (for pulsed systems) have interlocks designed to prevent firing of the laser by dumping the stored energy into a dummy load

·       (for continuous wave systems) have interlocks designed to turn off the power supply or interrupt the beam by means of shutters.

The beam path must be free of specularly reflective surfaces and combustible objects and the beam terminated in a non-combustible, non-reflective barrier or beam stop.

Warning Signs and Labels (top)

All Class 2, 3 and 4 laser equipment must be labeled indicating hazard classification, output power/energy, and lasing material or wavelength with words and symbols as indicated below:

• Class 2 laser equipment: CAUTION, Laser Radiation (or laser symbol), Do Not Stare Into Beam

• Class 3R laser equipment, below MPE: Danger, Laser Radiation (or laser symbol), Do Not Stare into Beam or View Directly with Optical Instruments

• Class 3R laser equipment, above MPE: DANGER, Laser Radiation (or laser symbol), Avoid Direct Eye Exposure

• Class 3B laser equipment: DANGER, Laser Radiation (or laser symbol), Avoid Direct Exposure to Beam

• Class 4 laser equipment: DANGER, Laser Radiation (or laser symbol), Avoid Eye or Skin Exposure to Direct or Scattered Radiation

Labels and warning signs should be displayed conspicuously in areas where they would best serve to warn individuals of potential safety hazards.  Normally, signs are posted at entryways to laser controlled areas and labels are affixed to the laser in a conspicuous location.

Table 5. Control Measures for the Four Laser Classes