1. The optical cavity contains the media to be excited with mirrors to redirect the produced photons back along the same general path.
   
   
2. The pumping system uses photons from another source as a xenon gas flash tube (optical pumping) to transfer energy to the media, electrical discharge within the pure gas or gas mixture media (collision pumping), or relies upon the binding energy released in chemical reactions to raise the media to the metastable or lasing state.
   
   
3. The laser medium can be a solid (state), gas, dye (in liquid), or semiconductor. Lasers are commonly designated by the type of lasing material employed.
   
   
   1. Solid state lasers have lasing material distributed in a solid matrix, e.g., the ruby or neodymium-YAG (yttrium aluminum garnet) lasers. The neodymium-YAG laser emits infrared light at 1.064 micrometers.
      
      
   2. Gas lasers (helium and helium-neon, HeNe, are the most common gas lasers) have a primary output of a visible red light. CO₂ lasers emit energy in the far-infrared, 10.6 micrometers, and are used for cutting hard materials.
      
      
   3. Excimer lasers (the name is derived from the terms excited and dimers) use reactive gases such as chlorine and fluorine mixed with inert gases such as argon, krypton, or xenon. When electrically stimulated, a pseudomolecule or dimer is produced and when lased, produces light in the ultraviolet range.
      
      
   4. Dye lasers use complex organic dyes like rhodamine 6G in liquid solution or suspension as lasing media. They are tunable over a broad range of wavelengths.
      
      
   5. Semiconductor lasers, sometimes called diode lasers, are not solid-state lasers. These electronic devices are generally very small and use low power. They may be built into larger arrays, e.g., the writing source in some laser printers or compact disk players.
   
   
   
4. The wavelength output from a laser depends upon the medium being excited. Table III:6-1 lists most of the laser types and their wavelength ouput defined by the medium being excited. Laser use today is not restricted to the laboratory or specialized industries. Table III:6-2 lists some of the major uses of lasers.

TABLE III:6-1. WAVELENGTHS OF MOST COMMON LASERS

Laser type Wavelength (µmeters) Laser type Wavelength (µmeters) Argon fluoride (Excimer-UV) Krypton chloride (Excimer-UV) Krypton fluoride (Excimer-UV) Xenon chloride (Excimer-UV) Xenon fluoride (Excimer-UV) Helium cadmium (UV) Nitrogen (UV) Helium cadmium (violet) Krypton (blue) Argon (blue) Copper vapor (green) Argon (green) Krypton (green) Frequency doubled       Nd YAG (green) Helium neon (green) Krypton (yellow) Copper vapor (yellow) 0.193 0.222 0.248 0.308 0.351 0.325 0.337 0.441 0.476 0.488 0.510 0.514 0.528 0.532 0.543 0.568 0.570 Helium neon (yellow) Helium neon (orange) Gold vapor (red) Helium neon (red) Krypton (red) Rohodamine 6G dye (tunable) Ruby (CrAlO3) (red) Gallium arsenide (diode-NIR) Nd:YAG (NIR) Helium neon (NIR) Erbium (NIR) Helium neon (NIR) Hydrogen fluoride (NIR) Carbon dioxide (FIR) Carbon dioxide (FIR) 0.594 0.610 0.627 0.633 0.647 0.570-0.650 0.694 0.840 1.064 1.15   1.504 3.39 2.70 9.6   10.6    Key:      UV   =   ultraviolet (0.200-0.400 µm)               VIS   =   visible (0.400-0.700 µm)               NIR   =   near infrared (0.700-1.400 µm)

TABLE III:6-2. MAJOR CATEGORIES OF LASER USE

Alignment Annealing Balancing Biomedical             Cellular research             Dental             Diagnostics             Dermatology             Ophthalmology             Surgery Communications Construction             Alignment             Ranging             Surveying Cutting Displays Drilling Entertainment Heat treating Holography Information handling             Copying             Displays             Plate making             Printing             Reading             Scanning             Typesetting             Videodisk             Marking Laboratory instruments Interferometry Metrology Plasma diagnostics Spectroscopy Velocimetry Lidar             Special photography             Scanning microscopy Military             Distance ranging             Rifle simulation             Weaponry Nondestructive training Scanning Sealing Scribing Soldering Welding

1. INDUSTRIAL HYGIENE. Potential hazards associated with compressed gases, cryogenic materials, toxic and carcinogenic materials and noise should be considered. Adequate ventilation shall be installed to reduce noxious or potentially hazardous fumes and vapors, produced by laser welding, cutting and other target interactions, to levels below the appropriate threshold limit values, e.g., American Conference of Governmental Industrial Hygienists (ACGIH) threshold limit values (TLV's) or Occupational Safety and Health Administration's (OSHA) permissible exposure limits (PEL's).
   
   
2. EXPLOSION HAZARDS. High-pressure arc lamps and filament lamps or laser welding equipment shall be enclosed in housings which can withstand the maximum pressures resulting from lamp explosion or disintegration. The laser target and elements of the optical train which may shatter during laser operation shall also be enclosed.
   
   
3. NONBEAM OPTICAL RADIATION HAZARDS. This relates to optical beam hazards other than laser beam hazards. Ultraviolet radiation emitted from laser discharge tubes, pumping lamps and laser welding plasmas shall be suitably shielded to reduce exposure to levels below the ANSI Z 136.1 (extended source), OSHA PEL's, and/or ACGIH TLV's.
   
   
4. COLLATERAL RADIATION. Radiation, other than laser radiation, associated with the operation of a laser or laser system, e.g., radio frequency (RF) energy associated with some plasma tubes, x-ray emission associated with the high voltage power supplies used with excimer lasers, shall be maintained below the applicable protection guides. The appropriate protection guide for RF and microwave energy is that given in the American National Standard "Safety levels with respect to human exposure to radio frequency electromagnetic fields, 300 kHz to 100 GHz," ANSI C95.1; the appropriate protection guides for exposure to X-ray emission is found in the Department of Labor Occupational Safety and Health Standards, 29 CFR Part 1910.1096 and the applicable State Codes. Lasers and laser systems which, by design, would be expected to generate appreciable levels of collateral radiation, should be monitored.
   
   
5. ELECTRICAL HAZARDS. The intended application of the laser equipment determines the method of electrical installation and connection to the power supply circuit (for example, conduit versus flexible cord). All equipment shall be installed in accordance with the National Electrical Code and the Occupational Safety and Health Act. [Additional specific recommendations can be found in Section 7.4 of ANSI Z 136.1 (1993)].
   
   
6. FLAMMABILITY OF LASER BEAM ENCLOSURES. Enclosure of Class IV laser beams and terminations of some focused Class IIIB lasers, can result in potential fire hazards if the enclosure materials are exposed to irradiances exceeding 10 W/cm². Plastic materials are not precluded as an enclosure material, but their use and potential for flammability and toxic fume release following direct exposure should be considered. Flame-resistant materials and commercially available products specifically designed for laser enclosures should also be considered.
   
   

1. EYE INJURY. Because of the high degree of beam collimation, a laser serves as an almost ideal point source of intense light. A laser beam of sufficient power can theoretically produce retinal intensities at magnitudes that are greater than conventional light sources, and even larger than those produced when directly viewing the sun. Permanent blindness can be the result.
   
   
2. THERMAL INJURY. The most common cause of laser-induced tissue damage is thermal in nature, where the tissue proteins are denatured due to the temperature rise following absorption of laser energy.
   
   
   1. The thermal damage process (burns) is generally associated with lasers operating at exposure times greater than 10 microseconds and in the wavelength region from the near ultraviolet to the far infrared (0.315 µm-103 µm). Tissue damage may also be caused by thermally induced acoustic waves following exposures to sub-microsecond laser exposures.
      
      
   2. With regard to repetitively pulsed or scanning lasers, the major mechanism involved in laser-induced biological damage is a thermal process wherein the effects of the pulses are additive. The principal thermal effects of laser exposure depend upon the following factors:
      
      
      • The absorption and scattering coefficients of the tissues at the laser wavelength. See Table III:6-1 for a summary of more common laser types and wavelengths.
        
        
      • Irradiance or radiant exposure of the laser beam.
        
        
      • Duration of the exposure and pulse repetition characteristics, where applicable.
        
        
      • Extent of the local vascular flow.
        
        
      • Size of the area irradiated.
        
        
3. OTHER.
   
   
   1. Other damage mechanisms have also been demonstrated for other specific wavelength ranges and/or exposure times. For example, photochemical reactions are the principal cause of threshold level tissue damage following exposures to either actinic ultraviolet radiation (0.200 µm-0.315 µm) for any exposure time or "blue light" visible radiation (0.400 µm-0.550 µm) when exposures are greater than 10 seconds.
      
      
   2. To the skin, UV-A (0.315 µm-0.400 µm) can cause hyperpigmentation and erythema.
      
      
   3. Exposure in the UV-B range is most injurious to skin. In addition to thermal injury caused by ultraviolet energy, there is the possibility of radiation carcinogenesis from UV-B (0.280 mm - 0.315 mm) either directly on DNA or from effects on potential carcinogenic intracellular viruses.
      
      
   4. Exposure in the shorter UV-C (0.200 µm-0.280 µm) and the longer UV-A ranges seems less harmful to human skin. The shorter wavelengths are absorbed in the outer dead layers of the epidermis (stratum corneum) and the longer wavelengths have an initial pigment-darkening effect followed by erythema if there is exposure to excessive levels. These biological effects are summarized in Table III:6-3.
      
      
   5. The hazards associated with skin exposure are of less importance than eye hazards; however, with the expanding use of higher-power laser systems, particularly ultraviolet lasers, the unprotected skin of personnel may be exposed to extremely hazardous levels of the beam power if used in an unenclosed system design.
      
      NOTE:   The primary purpose of an exiting laser beam, e.g. cutting or welding of hard materials, must not be forgotten! Some laser beams designed for material alteration may be effective some distance from their intended impact point.
      
      

 TABLE III:6-3. SUMMARY OF BASIC BIOLOGICAL EFFECTS OF LIGHT

Photobiological spectral domain Eye effects Skin effects Ultraviolet C (0.200-0.280 µm) Photokeratitis Erythema (sunburn) Skin cancer Ultraviolet B (0.280-315 µm) Photokeratitis Accelerated skin aging Increased pigmentation Ultraviolet A (0.315-0.400 µm) Photochemical UV cataract Pigment darkening Skin burn Visible (0.400-0.780 µm) Photochemical and thermal retinal injury Photosensitive reactions Skin burn Infrared A (0.780-1.400 µm) Cataract, retinal burns Skin burn Infrared B (1.400-3.00 µm) Corneal burn Aqueous flare IR cataract Skin burn Infrared C (3.00-1000 µm) Corneal burn only Skin burn

1. INTRODUCTION.
   
   
   1. The intent of laser hazard classification is to provide warning to users by identifying the hazards associated with the corresponding levels of accessible laser radiation through the use of labels and instruction. It also serves as a basis for defining control measures and medical surveillance.
      
      
   2. Lasers and laser systems received from manufacturers are required by federal law, 21 CFR Part 1000, to be classified and appropriately labeled by the manufacturer. It should be stressed, however, that the classification may change whenever the laser or laser system is modified to accomplish a given task.
      
      
   3. It should also be stressed that an agency such as the Food and Drug Administration's Center for Devices and Radiological Health (FDA/CDRH) does not "approve" laser systems for medical use. The manufacturer of the laser system first classifies the laser and then certifies that it meets all performance requirements of the Federal Laser Product Performance Standard (FLPPS). The forms submitted by the manufacturer to FDA/CDRH are reviewed for technical accuracy, omissions, and errors. If none are found, the manufacturer is notified only that the submission appears to be complete. Therefore, all lasers and laser systems that are manufactured by a company, or purchased by a company and relabeled and placed into commerce, or incorporated into a system and placed into commerce, shall be classified.
      
      
2. LASER HAZARD CLASSES.
   
   
   1. Virtually all of the U.S. domestic as well as all international standards divide lasers into four major hazard categories called the laser hazard classifications. The classes are based upon a scheme of graded risk. They are based upon the ability of a beam to cause biological damage to the eye or skin. In the FLPPS, the classes are established relative to the Accessible Emission Limits (AEL) provided in tables in the standard. In the ANSI Z 136.1 standard, the AEL is defined as the product of the Maximum Permissible Exposure (MPE) level and the area of the limiting aperture. For visible and near infrared lasers, the limiting aperture is based upon the "worst-case" pupil opening and is a 7 mm circular opening.
      
      
   2. Lasers and laser systems are assigned one of four broad Classes (I to IV) depending on the potential for causing biological damage. The biological basis of the hazard classes are summarized in Table III:6-4.
      
      a. Class I: cannot emit laser radiation at known hazard levels (typically continuous wave: cw 0.4 µW at visible wavelengths). Users of Class I laser products are generally exempt from radiation hazard controls during operation and maintenance (but not necessarily during service).
      
      Since lasers are not classified on beam access during service, most Class I industrial lasers will consist of a higher class (high power) laser enclosed in a properly interlocked and labeled protective enclosure. In some cases, the enclosure may be a room (walk-in protective housing) which requires a means to prevent operation when operators are inside the room.
      
      b. Class I.A.: a special designation that is based upon a 1000-second exposure and applies only to lasers that are "not intended for viewing" such as a supermarket laser scanner. The upper power limit of Class I.A. is 4.0 mW. The emission from a Class I.A. laser is defined such that the emission does not exceed the Class I limit for an emission duration of 1000 seconds.
      
      c. Class II: low-power visible lasers that emit above Class I levels but at a radiant power not above 1 mW. The concept is that the human aversion reaction to bright light will protect a person. Only limited controls are specified.
      
      d. Class IIIA: intermediate power lasers (cw: 1-5 mW). Only hazardous for intrabeam viewing. Some limited controls are usually recommended.
      
      NOTE:   There are different logotype labeling requirements for Class IIIA lasers with a beam irradiance that does not exceed 2.5 mW/cm² (Caution logotype) and those where the beam irradiance does exceed 2.5 mW/cm² (Danger logotype).
      
      e. Class IIIB: moderate power lasers (cw: 5-500 mW, pulsed: 10 J/cm² or the diffuse reflection limit, whichever is lower). In general Class IIIB lasers will not be a fire hazard, nor are they generally capable of producing a hazardous diffuse reflection. Specific controls are recommended.
      
      f. Class IV: High power lasers (cw: 500 mW, pulsed: 10 J/cm² or the diffuse reflection limit) are hazardous to view under any condition (directly or diffusely scattered) and are a potential fire hazard and a skin hazard. Significant controls are required of Class IV laser facilities.
   
   
   

   TABLE III:6-4. LASER CLASSIFICATIONS--SUMMARY OF HAZARDS

   Applies to --- wavelength ranges ---   ------------ Hazards ------------ Class UV VIS NIR  IR    Direct ocular Diffuse ocular Fire I X X X X   No No No IA -- X* -- --   Only after 1000 sec No No II -- X -- --   Only after 0.25 sec No No IIIA X X** X X   Yes No No IIIB X X X X   Yes Only when laser output is near Class IIIB limit of 0.5 Watt No IV X X X X   Yes Yes Yes Key: X  *   ** = = = Indicates class applies in wavelength range. Class IA applicable to lasers "not intended for viewing" ONLY. CDRH Standard assigns Class IIIA to visible wavelengths ONLY. ANSI Z 136.1 assigns Class IIIA to all wavelength ranges.

   
   
   
3. HOW TO DETERMINE THE CLASS OF LASERS DURING INSPECTION.
   
   
   1. The classification of a laser or laser product is, in some instances, a rather detailed process. It can involve determination of the AEL, measurement of the laser emission, measurement/determination of the emission pulse characteristics (if applicable), evaluation of various performance requirements (protective housing, interlocks, etc.) as specified by the FLPPS and/or ANSI standards.
      
      
   2. It should be stressed that classification is a required specification provided by the laser manufacturer and the label that specifies the class is found in only one location on the laser product. The class of the laser will be specified only on the lower left-hand corner (position three) of the warning logotype label.
      
      The logotype is the rectangular label that has the laser "sunburst" symbol and the warning statement of CAUTION (Class II and some Class IIIA) or DANGER (some Class IIIA, all Class IIIB and Class IV). This label will also have the type of laser designated (HeNe, Argon, CO₂, etc.) and the power or energy output specified (1 mW CW/MAX, 100 mJ pulsed, etc.).
      
      
   3. Class I lasers have no required labeling indicating the Class I status. Although the FLPPS requires no classification labeling of Class I lasers it does require detailed compliance with numerous other performance requirements (i.e., protective housing, identification and compliance labeling, interlocking, etc.)
   
   
   
4. ANSI Z 136.2 OPTICAL FIBER SERVICE GROUP DESIGNATIONS.
   
   
   1. Optical Fiber Communication Systems (OFCS) and the associated optical test sets use semiconductor lasers or LED transmitters that emit energy at wavelengths typically in the range from 0.650 to 1.20 mm into the light-guide fiber-optic cables.
      
      
   2. All OFCS are designed to operate with the beam totally enclosed within the fiber-optic and associated equipment and, therefore, are always considered as Class I in normal operation.
      
      
   3. The only risk for exposure would occur during installation and service when light-guide cables are disconnected or during an infrequent accidental cable break.
      
      
   4. Under the requirements of the ANSI Z 136.2 (1988) Standard "For the Safe Use of Optical Fiber Communication Systems Utilizing Laser Diode and LED Sources," Optical Fiber Communication Systems (OFCS) are assigned into one of four service group (SG) designations: SG1, SG2, SG3a, SG3b, depending on the potential for an accessible beam to cause biological damage.
      
      
   5. The service group designations relate to the potential for ocular hazards to occur only during accessible beam conditions. This would normally occur only during periods of service to a OFCS. Such designations apply only during periods of service in one of the following four service groups:
      
      a. Service Group 1: An OFCS that is SG1 has a total output power that is less than the Accessible Emission Limit (AEL) for Class I and there is no risk of exceeding the Maximum Permissible Irradiance (MPI) when viewing the end of a fiber with a microscope, an eye-loupe or with the unaided eye.
      
      b. Service Group 2: An OFCS is SG2 only if wavelengths between 0.400 and 0.700 mm are emitted and is potentially hazardous if viewed for more than 0.25 second. (Note: At present there are virtually no OFCS's that operate in this wavelength range.)
      
      c. Service Group 3A: A SG 3A OFCS is not hazardous when viewed with the unaided eye and is hazardous only when viewed with a microscope or an eye-loupe.
      
      d. Service Group 3B: OFCS that meet none of the above criteria are designated as SG 3B.
      
      NOTE:   OFCS's where the total power is at or above 0.5W do not meet the criteria for optical fiber service group designation. In this case, the OFCS's are treated as a standard laser system.
   
   
   

1. REQUIREMENTS OF LASER STANDARDS. In the United States, several organizations concern themselves with laser safety. These organizations include the American National Standards Institute (ANSI); the Center for Devices and Radiological Health (CDRH) of the Food and Drug Administration (FDA); the Department of Labor's Occupational Safety and Health Administration (OSHA); and the Council of Radiation Control Program Directors (CRCPD). Several state governments and the CRCPD have developed a model state standard for laser safety.
   
   
   1. OSHA Regulatory Practice. At the present time, OSHA does not have a comprehensive laser standard, though 29 CFR 1926.54 is applicable to the construction industry. A standard for personal protective equipment (Subpart I) may apply in some cases.
      
      The construction standard 29 CFR 1926.102(b)(2), for eye and face protection, states that "employees whose occupation or assignment requires exposure to laser beams shall be furnished suitable laser safety goggles which will protect for the specific wavelength of the laser and be of optical density (O.D.) adequate for the energy involved."
      
      OSHA citations are issued by invoking the general duty clause or, in some cases, Subpart I. In such cases, the employers are required to revise their reportedly unsafe work place using the recommendations and requirements of such industry consensus standards as the ANSI Z 136.1 Standard. See also Table III:6-8.
      
      
   2. Specific and Model State Laser Regulations. A few states currently have laser regulations. Requirements are generally concerned with the registration of lasers and the licensing of operators and institutions. Physician-used and other medical lasers are generally exempt from state requirements.
      
      The complexity of state laser regulations may change in the future pending adoption of the "Suggested State Regulation for Lasers" promulgated through the Conference of Radiation Control Program Directors. This model state standard has been adopted in part, for example, by Arizona and Florida. Several other states have enacted some form of regulation. Table III:6-5 summarizes state regulations.
      
      

      TABLE III:6-5. SUMMARY OF CURRENT STATE LASER REGULATIONS

      State Department Regulation Alaska Arizona* Arkansas   Florida*   Georgia Illinois Massachusetts Montana New York Pennsylvania Texas Washington Environmental Conservation Radiation Regulatory Agency Division of Radiation Control   & Emergency Management Department of Health &   Rehabilitative Services Department of Public Health Department of Nuclear Safety Department of Public Health Health & Environmental Services Department of Labor Environmental Resources Department of Health Labor & Industry Title 18, Article 7     Act 460 Non-Ionizing Chapter: 10D-89 Chapter: 290-5-27 Chapter: 111 ½ 105 CMR 21 92-003 Code Rule 50 Chapter: 203, Title 25 Radiation Control Act Parts 50, 60, 70 Chapter 296-62-WAC * Using CRCPD "Model State" laser standard as basis.

      
      
      
   3. FDA Center for Devices and Radiological Health Performance Requirements.
      
      a. The CDRH of the Department of Health and Human Services was chartered by Congress to standardize the manufacture of lasers in interstate commerce after August 2, 1976. CDRH also has the responsibility for enforcing compliance with the medical devices legislation. All manufacturers of surgical lasers must obtain premarket approval of their devices through the CDRH.
      
      b. FDA sanctions the exploratory use of lasers for specific procedures through a process known as an Investigational Device Exemption (IDE). Approval of an IDE permits the limited use of a laser expressly for the purpose of conducting an investigation of the laser's safety and effectiveness. Once an IDE has been prepared and approved by the CDRH, the manufacturer may then actively market the laser for that specific medical or surgical procedure.
      
      c. The FDA/CDRH Federal Laser Product Performance Standard (FLPPS) regulates the manufacturer of commercial laser products, not the user. The standard does not contain specific design specifications, but is a conceptual, performance standard which the designer of laser products must consider. The intent is to insure laser product safety.
      
      d. FLPSS is applicable to lasers or laser systems sold by a company within or imported into the U.S. In some cases it can also apply when a laser or laser system is transferred within a company for internal use within the U.S. The compliance procedure requires implementation of the procedures and requirements as set forth in the U.S. Federal Laser Product Performance Standard: 21 CFR Part 1000 [parts 1040.10 and 1040.11].
      
      e. Under the requirements of the FLPPS, the manufacturer is first required to classify the laser as either a Class-I, Class-II, Class-I.A., Class-IIIA, Class-IIIB, or Class-IV laser product and then to certify (by means of a label on the product) as well as submit a report demonstrating that all requirements (performance features) of the compliance standard are met. Specific performance features include:
      
      
      • protective housing;
      • protective housing warning labels and logotype labels;
      • product identification label and certification statement;
      • safety interlocks;
      • emission indicator;
      • remote interlock connector;
      • key control;
      • beam attenuator;
      • specification of control locations;
      • viewing optic limitations;
      • scanning beam safeguards; and
      • manual reset of beam cutoff.
      
      
      f. FDA/CDRH performance requirements are tabulated in Appendix III:6-1. An outline to assist in evaluating FLPPS laser system performance requirements is included in Appendix III:6-2.
      
      
   4. The American National Standard Institute (ANSI). An American National Standard implies a consensus of those substantially concerned with its scope and provisions. These standards are intended as a guide for manufacturers, consumers, and the general public. However, there is no inherent requirement for any person or company to adhere to an ANSI standard. Compliance is voluntary unless specifically required by an organization. For example, the U.S. Department of Energy requires adherence to the ANSI Z 136.1 by their staff as well as by all contractor organizations. Appendix III:6-3 summarizes ANSI Standards applicable to laser safety.
   
   
   
2. LASER EXPOSURE LIMITS. At present either the FDA criteria for medical lasers or the following ANSI standards can be useful in evaluating laser safety.
   
   
   1. FDA Long-Term Exposure Limits. The FDA/CDRH Federal Laser Product Performance Standard (FLPPS) assumes a linearly additive biological effect for exposures to visible light between 10 and 10^4 seconds (2.8 hours). The standard accepts that a cumulative radiant energy exposure of 3.85 millijoules (mJ) will not cause a biological effect. Hence a 10-second total accumulated exposure corresponds to an average power entering a 7-mm aperture of 385 microwatts (µW). For an exposure of 10^4 seconds, the average power would be 0.385 µW. In the FLPPS, the power level of 0.385 µW is referred to as the Class I Accessible Emission Limit (AEL) for a visible CW laser.
      
      
   2. ANSI Z 136.1, Long-Term Exposure Limits.
      
      a. The ANSI Z 136.1 (1993) standard is a "user" standard and therefore provides maximum permissible exposure (MPE) limits. These were derived by normalizing the power (or pulse energy) data derived from biological research studies relative to a defined limiting aperture. For example, in the visible and near-infrared spectra, the limiting aperture is based upon the diameter of a fully dilated pupil of the human eye, 7 mm. The area of a 7-mm pupil is 0.385 cm². Hence, the irradiance limit for long-term ocular exposure is computed by dividing the AEL value of 0.385 µW by the area of the limiting aperture of 0.385 cm². This yields the worst-case MPE value of 1.0 µW/cm² for long-term exposure in the wavelength range of 0.400 to 0.550 mm.
      
      b. The ANSI Z 136 and FDA/CDRH allowable-exposure limits for CW lasers (Class I limits) are essentially identical for wavelengths between 0.400 and 0.550 µm. The ANSI limits are, however, more relaxed for wavelengths between 0.550 and 1.40 µm. ANSI recognizes a decreased biological hazard in the red and infrared regions that is not recognized by the CDRH.
      
      c. The ANSI Z 136 MPE level for a very long term exposure by a helium-neon laser is, in fact, seventeen times greater than the CDRH standard. In the 1976 revision, ANSI Z 136 introduced the correction factor CB which has a value of 17.5 at the 0.633-µm HeNe laser wavelength, and, thus, permitted a radiant exposure of 185 mJ/cm² accumulated exposure for times from T₁ = 453 seconds to 104 seconds, and about 18 w/cm² (7 w in a 7-mm limiting aperture) for continuous operation of exposure durations exceeding 104 seconds.
      
      
   3. ANSI Z 136.1, Repetitively Pulsed Exposures.
      
      a. The ANSI Z 136 standard requires a decrease in the maximum permissible exposure (MPE) for scanned or repetitive-pulse radiation as compared to continuous-wave radiation for pulse repetition frequencies (PRF) in the general range of 1000-15000 Hz. Because of pulse additivity, scanned or repetitively pulsed radiation with repetition rates less than 15 KHz have lower retinal damage threshold levels than CW radiation of comparable power.
      
      b. The ANSI Z 136 Standard includes a reduction factor of the threshold for each of the single pulses based on biological data that are not yet well explained by any theory. The FDA/CDRH standard does not recognize this repetitive-pulse correction factor. However, some experts envision the possibility of a repetitively pulsed laser which is Class I by the FDA/CDRH standard could be rated Class II or even Class IIIB by the ANSI Z 136 standard.
      
      c. The ANSI standard requires that multiple-pulse (scanning) lasers operating from 1 to 15,000 Hz have a correction to the single pulse MPE. The correction factor is determined by taking the fourth root of the total number of pulses (N) in a pulse train. Then, the correction factor is calculated such that the MPE radiant exposure or integrated radiance of an individual pulse within the train is reduced by a factor N^-¼.
      
      
   4. ANSI Z 136.1, Maximum Permissible Exposure Limits.
      
      a. A summary of Maximum Permissible Exposure (MPE) limits for direct ocular exposures for some of the more common lasers is presented in Table III:6-6. For further information on MPE values, refer to the ANSI Z 136.1 "Safe Use of Lasers" Standard.
      
      b. The information in Table III:6-6 provides the MPE value for different lasers operating for different overall exposure times. The times chosen were:
      
      
      • 0.25 second: The human aversion time for bright-light stimuli (the blink reflex). Thus, this becomes the "first line of defense" for unexpected exposure to some lasers and is the basis of the Class II concept.
      • 10 seconds: The time period chosen by the ANSI Z 136.1 committees represents the optimum "worst-case" time period for ocular exposures to infrared (principally near-infrared) laser sources. It was argued that natural eye motions dominate for periods longer than 10 seconds.
      • 600 seconds: The time period chosen by the ANSI Z 136.1 committees represents a typical worst-case period for viewing visible diffuse reflections during tasks such as alignment.
      • 30,000 seconds: The time period that represents a full 1-day (8-hour) occupational exposure. This results from computing the number of seconds in 8 hours; e.g.: 8 hours × 60 minutes/hour × 60 seconds/minute = 28,800 seconds. Rounded off, it becomes 30,000 seconds.
      
      
      c. The "safety" exposure limits (MPE's) in Table III:6-6 are expressed in irradiance terms (W/cm²) that would be measured at the cornea. Note that they vary by wavelength and exposure time.
      
      

   TABLE III:6-6. SUMMARY: MAXIMUM PERMISSIBLE EXPOSURE LIMITS*

   Wave- length -------------- MPE level (W/cm2) -------------- Laser type (µm) 0.25 sec 10 sec 600 sec 30,000 sec CO2 (CW) 10.6     --- 100.0 × 10-3 --- 100.0 × 10-3 Nd: YAG (CW) 1.33   --- 5.1 × 10-3 --- 1.6 × 10-3 Nd: YAG (CW) 1.064 --- 5.1 × 10-3 --- 1.6 × 10-3 Nd: YAG  (Q-switched) 1.064 --- 17.0 × 10-6 --- 2.3 × 10-6 GaAs (Diode/CW) 0.840 --- 1.9 × 10-3 --- 610.0 × 10-6 HeNe (CW) 0.633 2.5 × 10-3 --- 293.0 × 10-6 17.6 × 10-6 Krypton (CW) 0.647 0.568 0.530 2.5 × 10-3 31.0 × 10-6 16.7 × 10-6 --- --- --- 364.0 × 10-6 2.5 × 10-3 2.5 × 10-3 28.5 × 10-6 18.6 × 10-6 1.0 × 10-6 Argon (CW) 0.514 2.5 × 10-3 --- 16.7 × 10-6 1.0 × 10-6 XeFl (Excimer/ CW) 0.351 --- --- --- 33.3 × 10-6 XeCl (Excimer/ CW) 0.308 --- --- --- 1.3 × 10-6

   * Source: ANSI Z 136.1 (1993)
   
   
3. LASER HAZARD COMPUTATIONS.
   
   
   1. NHZ Definition, Use, and Values.
      
      a. The Nominal Hazard Zone (NHZ) describes the space within which the level of direct, reflected, or scattered radiation during normal operation exceeds the MPE. The NHZ associated with open-beam Class IIIB and Class IV laser installations can be useful in assessing area hazards and implementing controls.
      
      b. It is often necessary in some applications where open beams are required (e.g., industrial processing, laser robotics, surgical uses) to define the area where the possibility exists for potentially hazardous exposure. This is done by determining the NHZ. Consequently, persons outside the NHZ boundary would be exposed below the MPE level and are considered to be in a non-hazardous location.
      
      c. The NHZ boundary may be defined, for example, by direct beams (intrabeam) and diffusely scattered laser beams, as well as beams transmitted from fiber optics and/or through lens arrays. The NHZ perimeter is the envelope of MPE exposure levels from any specific laser installation geometry.
      
      d. The purpose of an NHZ evaluation is to define that space where control measures are required. This is an important factor since, as the scope of laser uses has expanded, controlling lasers by total enclosure in a protective housing or interlocked room is limiting and, in many instances, an expensive overreaction to the real hazards. The following factors are required in NHZ computations:
      
      
      • laser power or energy output;
      • beam diameter;
      • beam divergence;
      • pulse repetition frequency (prf) (if applicable);
      • wavelength;
      • beam optics and beam path; and
      • maximum anticipated exposure duration.
      
      
      e. Note that the ANSI Z 136 MPE value is required in all NHZ calculations. Examples of NHZ calculations can be found in the appendix of ANSI Z 136.1 (1993). In addition, computer software is also available to assist in the computations for NHZ, optical densities of protective eye wear, and other aspects of laser hazard analysis.
      
      
   2. NHZ Example Summary. The intrabeam (direct) hazard for a Nd:YAG laser extends from 792 meters to 1410 meters, depending upon whether a 10-second or 8-hour criterion is used, as summarized in Table III:6-7. Similarly, with a lens on the laser, the hazard for a Nd:YAG laser exists over a range from 6.3 meters to 11.3 meters. The diffuse reflection zone for this laser type is, however, markedly smaller, 0.8 meter to 1.4 meters. Nonetheless, the analysis suggests that operating personnel and support staff close to the laser still need eye protection even for diffuse reflections.
      
      Other calculations are also presented in Table III:6-7 for a 500-Watt CO₂ and a 5-Watt argon laser. Note that the NHZ's do not vary for the CO₂ laser (because the MPE values are nearly identical for 10-second and 8-hour criteria). Also note that the diffuse reflection NHZ's are very small except for the 8-hour criterion for the argon laser. In most cases, 0.25 second can be used with visible lasers unless intentional staring is required or intended.

Source: ANSI A 136.1 (1993)

1. INTRABEAM OPTICAL DENSITY DETERMINATION.
   
   
   1. Based upon these typical exposure conditions, the optical density required for suitable filtration can be determined. Optical density (OD) is a logarithmic function defined by:
      
      
      EQUATION III:6-1. OPTICAL DENSITY
      

      OD  =  log10   H 0   MPE

      
      
      

      where:
      	H0 MPE	=   =	Anticipated worst-case exposure (J/cm2 or W/cm2) Maximum permissible exposure level expressed in the same units as H0

      
      
      
   2. Based upon the worst case exposure conditions outlined above, one can determine the optical density recommended to provide adequate eye protection for this laser. For example, the minimum optical density at the 0.514 µm argon laser wavelength for a 600-second direct intrabeam exposure to the 5-watt maximum laser output can be determined as follows:
      
      

      Where:
      f	=   5 Watts
      MPE	=   16.7 W/cm2 (using 600-second criterion)
      d	=   7 mm (worst-case pupil size)
      Computing the worst-case exposure H0:
      H0	=   [Power/Area]  =  f/A  =  4f/ pd2
      H0	=   [(4)(5.0)/ p(0.7)2]
      H0	=   12.99 W/cm2
      Substitution gives:
      OD	=   log10 [(12.99)/(16.7 × 10-6)]
      OD	=   5.9

      
      
      
   3. The most conservative approach would be to choose an 8-hour (occupational) exposure. In this case, the optical density at 0.514 µm is increased to OD = 7.1 for a 5.0-watt intrabeam exposure because the 8-hour (30,000 §) MPE is reduced to 1.0 × 10^-6 W/cm². The OD values for various lasers, computed for various appropriate exposure times, are presented in Table III:6-8. It should be stressed these values are for intrabeam viewing (worst case) only. Viewing Class IV diffuse reflections (such as during alignment tasks) require, in general, less OD. These should be determined for each situation and would be dependent upon the laser parameters and viewing distance.

TABLE III:6-9. ENGINEERING CONTROL MEASURES FOR THE FOUR LASER CLASSES [ANSI Z 136.1 (1993)]

------------------ Class ----------------- Control measures I IA II IIIA IIIB IV Protective housing X X X X X X Without protective housing -- LSO shall establish alternate controls -- Interlocks on protective housing a a a X X X Service access panel b b b b b X Key switch master _ _ _ _ · X Viewing portals _ _ Collecting optics _ _ Totally open beam path _ _ _ _ X X Limited open beam path _ _ _ _ X X Remote interlock connector _ _ _ _ · X Beam stop or attenuator _ _ _ · · X Activation warning system _ _ _ _ · X Emission delay _ _ _ _ _ · Class IIIB laser controlled area _ _ _ _ X _ Class IV laser controlled area _ _ _ _ _ X Laser outdoor controls _ _ _ _ X X Temporary laser controlled area b b b b _ _ Remote firing & monitoring _ _ _ _ _ · Labels _ X X X X X Area posting _ _ · · X X Administrative & procedural controls _ X X X X X Standard operating procedures _ _ _ _ · X Output emission limitations _ _ _ --LSO determines-- Education and training _ _ _ X X X Authorized personnel _ _ _ _ X X Alignment procedures _ _ X X X X Eye protection _ _ _ _ · X Spectator control _ _ _ _ · X Service personnel b b b b X X Laser demonstration _ _ X X X X Laser fiber optics _ _ X X X X

Key:            X a. b. _  ·  =   =   =   =   =   =  Shall. Shall if embedded Class IIIA, Class IIIB, Class IV. Shall if embedded Class IIIB or Class IV. No requirement. Should. Shall if MPE is exceeded.

1. CONTROL MEASURES: OVERVIEW.
   
   
   1. There are four basic categories of controls useful in laser environments. These are engineering controls, personal protective equipment, administrative and procedural controls, and special controls. The controls to be reviewed here are based upon the recommendations of the ANSI Z 136.1 standard.
      
      
   2. Important in all controls is the distinction between the functions of operation, maintenance, and service. First, laser systems are classified on the basis of level of the laser radiation accessible during operation. Maintenance is defined as those tasks specified in the user instructions for assuring the performance of the product and may include items such as routine cleaning or replenishment of expendables. Service functions are usually performed with far less frequency than maintenance functions (e.g., replacing the laser resonator mirrors or repair of faulty components) and often require access to the laser beam by those performing the service functions. The safety procedures required for such beam access during service functions should be clearly delineated in the laser product's service manual.
   
   
   
2. LASER SAFETY OFFICER (LSO).
   
   
   1. The LSO has the authority to monitor and enforce the control of laser hazards and effect the knowledgeable evaluation and control of laser hazards. The LSO administers the overall laser safety program where the duties include, but are not limited to, items such as confirming the classification of lasers, doing the NHZ evaluation, assuring that the proper control measures are in place and approving substitute controls, approving standard operating procedures (SOP's), recommending and/or approving eye wear and other protective equipment, specifying appropriate signs and labels, approving overall facility controls, providing the proper laser safety training as needed, conducting medical surveillance, and designating the laser and incidental personnel categories.
      
      
   2. The LSO should receive detailed training including laser fundamentals, laser bioeffects, exposure limits, classifications, NHZ computations, control measures (including area controls, eye wear, barriers, etc.), and medical surveillance requirements.
      
      
   3. In many industrial situations, the LSO functions will be a part-time activity, depending on the number of lasers and general laser activity. The individual is often in the corporate industrial hygiene department or may be a laser engineer with safety responsibility. Some corporations implement an internal laser policy and establish safety practices based upon the ANSI Z 136.1 standard as well as their own corporate safety requirements.
   
   
   
3. CLASS I, CLASS II, CLASS I.A., AND CLASS IIIA LASERS. Accident data on laser usage have shown that Class I, Class II, Class I.A., and Class IIIA lasers are normally not considered hazardous from a radiation standpoint unless illogically used.
   
   

   Direct exposure on the eye by a beam of laser light should always be avoided with any laser, no matter how low the power.

   
   
   

4. BEAM PATH CONTROLS. There are some uses of Class IIIB and Class IV lasers where the entire beam path may be totally enclosed, other uses where the beam path is confined by design to significantly limit access and yet other uses where the beam path is totally open. In each case, the controls required will vary as follows:
   
   
   1. Enclosed (Total) Beam Path.
      
      a. Perhaps the most common form of a Class I laser system is a high-power laser that has been totally enclosed (embedded) inside a protective enclosure equipped with appropriate interlocks and/or labels on all removable panels or access doors. Beam access is prevented, therefore, during operation and maintenance.
      
      b. Such a completely enclosed system, if properly labeled and properly safeguarded with protective housing interlocks (and all other applicable engineering controls), will fulfill all requirements for a Class I laser and may be operated in the enclosed manner with no additional controls for the operator.
      
      c. It should be noted that during periods of 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 will not alter the Class I status of the laser during operation.
      
      
   2. Limited Open Beam Path.
      
      a. It is becoming an accepted work practice, particularly with industrial materials-processing lasers, to build an enclosure that completely surrounds the laser-focusing optics and the immediate area of the workstation. Often a computer-controlled positioning table is located within this enclosure. The design often allows a gap of less than one quarter of an inch between the bottom of the enclosure and the top of the material to be laser processed. Such a design enables the part to be laser cut or welded to move while the laser delivery optics remain stationary.
      
      b. Such a system might not meet the stringent "human access" requirements of the FLPPS for a Class I laser, but the real laser hazards are well confined. Such a design provides what can be called a limited open beam path. In this situation, the ANSI Z 136.1 standard recommends that the LSO shall conduct a laser hazard analysis and establish the extent of the NHZ.
      
      c. In many system designs, (such as described above), the NHZ will be extremely limited, and procedural controls (rather than elaborate engineering controls) will be sufficient to ensure safe use. In many cases, the laser units may be reclassified by the LSO as Class I under the specifications of the ANSI Z 136 standard.
      
      d. Such an installation will require a detailed standard operating procedure (SOP). Training is also needed for the system operator commensurate with the class of the embedded laser.
      
      e. Protective equipment (eye protection, temporary barriers, clothing and/or gloves, respirators, etc.) would be recommended, for example, only if the hazard analysis indicated a need or if the SOP required periods of beam access such as during setup or infrequent maintenance activities. Temporary protective measures for service can be handled in a manner similar to the service of any embedded Class IV laser.
      
      
   3. Totally Unenclosed Beam Path. There are several specific application areas where high power (Class IIIB and Class IV) lasers are used in an unenclosed beam condition. This would include, for example, open industrial processing systems (often incorporating robotic delivery), laser research laboratory installations, surgical installations, etc. Such laser uses will require that the LSO conduct a hazard analysis and NHZ assessment. Controls are chosen to reflect the magnitude of hazards associated with the accessible beam.
   
   
   
5. LASER-CONTROLLED AREA. When the entire beam path from a Class IIIB or Class IV laser is not sufficiently enclosed and/or baffled to ensure that radiation exposures will not exceed the MPE, a "laser-controlled area" is required. During periods of service, a controlled area may be established on a temporary basis. The controlled area will encompass the NHZ. Those controls required for both Class IIIB and Class IV installations are as follows:
   
   
   1. Posting with Appropriate Laser Warning Signs.
      
      a. Class IIIA (beam irradiance 2.5 mW/cm²), Class IIIB and Class IV lasers: Require the ANSI DANGER sign format: white back-ground, red laser symbol with black outline and black lettering (see Appendix III:6-4). Note that under ANSI Z 136.1 criteria, area posting is required only for Class IIIB and Class IV lasers.
      
      b. Class II or Class IIIA areas (if area warning is deemed unnecessary by the LSO): All signs (and labels) associated with these lasers (when beam irradiance for Class IIIA does not exceed 2.5 mW/cm²) use the ANSI CAUTION format: yellow background, black symbol and letters.
      
      c. During times of service and other times when a temporary laser-controlled area is established, an ANSI NOTICE sign format is required: white background, red laser symbol with blue field and black lettering. This sign is posted only during the time when service is in progress. Examples of area warning signs and logotype designs are given in Appendix III:6-4.
      
      
   2. Operated by Qualified and Authorized Personnel. Training of the individuals in aspects of laser safety is required for Class IIIB and Class IV laser installations.
      
      
   3. Transmission from Indoor Controlled Area. The beams shall not, under any circumstances, be transmitted from an indoor laser-controlled area unless for specific purposes (such as testing). In such cases, the operator and the LSO must assure that the beam path is limited to controlled air space.
   
   
   
6. CLASS IV LASER CONTROLS--GENERAL REQUIREMENTS. Those items recommended for Class IIIB but required for Class IV lasers are as follows:
   
   
   • Supervision directly by an individual knowledgeable in laser safety.
   • Entry of any noninvolved personnel requires approval.
   • A beam stop of an appropriate material must be used to terminate all potentially hazardous beams.
   • Use diffusely reflecting materials near the beam, where appropriate.
   • Appropriate laser protective eye wear must be provided all personnel within the laser controlled area.
   • The beam path of the laser must be located and secured above or below eye level for any standing or seated position in the facility.
   • All windows, doorways, open portals, etc., of an enclosed facility should be covered or restricted to reduce any escaping laser beams below appropriate ocular MPE level.
   • Require storage or disabling of lasers when not in use.
   
   
   
7. ENTRYWAY CONTROL MEASURES (CLASS IV). In addition, there are specific controls required at the entryway to a Class IV laser controlled area. These can be summarized as follows:
   
   
   • All personnel entering a Class IV area shall be adequately trained and provided proper laser protective eye wear.
   • All personnel shall follow all applicable administrative and procedural controls.
   • All Class IV area and entryway controls shall allow rapid entrance and exit under all conditions.
   • The controlled area shall have a clearly marked "Panic Button" (nonlockable disconnect switch) that allows rapid deactivation of the laser.
   
   
   Class IV areas also require some form of area and entryway controls. In the past, doorway interlocking was customary for Class IV installations. The ANSI Z 136 Standard now provides four options that allow the LSO to provide an entryway control suited for the installation. The options include:
   
   
   1. Nondefeatable Entryway Controls. A nondefeatable control, such as a magnetic switch built into the entryway door which cuts the beam off when the door is opened, is one option. In this case, training is required only for those persons who regularly work in the laser area.
      
      
   2. Defeatable Entryway Controls.
      
      a. Defeatable controls may be used at an entryway, for example, during long-term testing in a laser area. In this case the controls may be temporarily made inactive if it is clearly evident that there is no hazard at the point of entry. Training is required for all personnel who may frequently require entry into the area.
      
      b. Such defeatable controls shall be designed to allow both rapid egress by the laser personnel at all times and admittance to the laser controlled area in an emergency condition. A readily accessible "panic button" or control/disconnect switch shall be available for deactivating the laser under such emergency conditions.
      
      c. Under conditions where the entire beam path is not completely enclosed, access to the laser-controlled area shall be limited only to persons wearing proper laser protective eye wear when the laser is capable of emitting a beam. In this case, all other optical paths (for example, windows) from the facility shall be covered or restricted in such a way as to reduce the transmitted intensity of the laser radiation to levels at or below the MPE for direct irradiation of the eye.
      
      
   3. Procedural Entryway Controls. A blocking barrier, screen, or curtain that can block or filter the laser beam at the entryway may be used inside the controlled area to prevent the laser light from exiting the area at levels above the applicable MPE level. In this case, a warning light or sound is required outside the entryway that operates when the laser is energized and operating. All personnel who work in the facility shall be appropriately trained.
      
      
   4. Entryway Warning Systems. In order to safely operate a Class IV laser or laser system, a laser warning system shall be installed as described:
      
      
      • A laser activation warning light assembly shall be installed outside the entrance to each laser room facility containing a Class IV laser or laser system.
      • In lieu of a blinking entryway warning, the entryway light assembly may alternatively be interfaced to the laser in such a manner that a light will indicate when the laser is not operational (high voltage off) and by an additional light when the laser is powered up (high voltage applied) but not operating and by an additional (flashing) light when the laser is operating.
        
        
      A laser warning sign shall be posted both inside and outside the laser-controlled area.
   
   
   
8. TEMPORARY LASER-CONTROLLED AREA. Should overriding interlocks become necessary during periods of special training, service, or maintenance, and access to Class IIIB or Class IV lasers is required, a temporary laser-controlled area shall be devised following specific procedures approved by the LSO. These procedures shall outline all safety requirements necessary during such operation.