Laser or Laser System Classification and Risk
Bruce E. Stuck, TSC-1 Chair
The ANSI Z136 series of documents provide practical guidance for the safe use of lasers and laser systems in many diverse configurations and environments. Classification of laser systems on a scale from Class 1 to Class 4 offers the purchaser, recipient or new user a rapid assessment of the relative hazard or risk to personnel. The laser Class scale is hierarchical and range from Class 1 systems that pose little or no risk to personnel to Class 4 systems that are hazardous with significant risk to personnel. General physical and administrative control measures are associated with each Class to mitigate or minimize the associated risk. Physical control measures include labels and signage, warning lights, room entry and laser cabinet interlocks and key systems, eye protection and protective barriers, rapid shut-off switches and fail-safe procedures. Administrative control measures outline operator, user, and ancillary personnel training requirements, use standard operating procedures (SOPs), medical surveillance recommendations, reporting procedures and laser safety officer (LSO) requirements and responsibilities. The higher the laser Class, the greater the number of both administrative and physical control measures. Laser or laser system classification is very essential to an effective laser safety program to minimize radiation hazards (or risk) by providing indicators of the relative risk (Classification) and mitigating procedures (Control Measures).
Laser/Laser System Classification, Maximum Permissible Exposures and Laser Bioeffects
There are seven levels of laser classification that include Class 1, Class 1M, Class 2, Class 2M, Class 3R, Class 3B, and Class 4. The details for classification are given in Section 3 and the associated tables in Z136.1-2014 (Ref 1). The Classification is based on the emission characteristics of the laser system, the configuration, and conditions of use. For classification, the emission characteristics are compared to selected maximum permissible exposures (MPE) to assess the relative hazard or risk (Section 8). The MPEs are promulgated by ANSI Z136.1-2014 (Ref 1) and expressed in terms of the exposure dose (radiant exposure in J×cm-2) or exposure dose rate (irradiance in W×cm-2) incident on the eye and skin. These exposure limits (MPEs) are a function of the exposure conditions that include wavelength, exposure duration, the irradiance diameter or “spot” size, and include assessment methods to evaluate repetitive or repeated exposures. The MPEs are anchored to a laser bioeffects data describing dose-response relationships and/or threshold laser-induced injury (Ref 2, 3, 4). In general, the MPE is defined as the “level of laser radiation to which an unprotected person may be exposed without adverse biological change in the eye or skin” (Ref 1).
Biological thresholds (Ref. 2, 3, 4) for agreed-upon response criteria coupled with understanding of the light-tissue interaction mechanisms are used to determine the MPEs. The biological response criteria or threshold can include endpoints that range from visual observations to cellular effects by histo- or cyto-chemical assays. The eye is an imaging system with a large dynamic range to support vision from bright daylight to dark night. Because of its sensitivity and ability to focus collimated radiation to a small retinal image, the eye is susceptible to laser-induced injury at relatively low doses. In addition, laser-induced veiling and disability glare or dazzle may cause temporary visual performance dysfunction in the absence of a permanent effect on the retina. Although these effects may be considered “adverse” if performing a visual demanding task, the dose and required conditions to produce disturbing glare sensations may be below the MPE. If only the skin is exposed, a minimally visible erythema may be used as the endpoint to establish the exposure limit. Induction of an aversion (i.e. “look” away or movement “out” of the beam) may minimize the localized beam dwell time on a given tissue area and therefore reduce the dose below injury threshold or exposure limit. Both acute and chronic effects are assessed to determine if the exposure dose results in an “adverse” effect. If the biological effect is measured in a cellular or animal model, the relative susceptibility of human exposure must be considered. Delayed effects are observed by assessing the endpoint at extended observation times (i.e. time after exposure). The dose requirements for different endpoints are compared (e.g., an ophthalmoscopically visible lesion threshold dose may be compared with a threshold cellular response by histopathological methods). Recovery, repair rates, the slope the dose-response curve for a given exposure condition, and the ability of the dose to induce an aversion response are considered when assigning an exposure limit (or MPE). Some studies of biological effects have been reproduced in several laboratories. Exposure data are discussed, plotted, and analyzed (Ref 4) to define the MPEs based upon the functional relationships between a threshold biological effect and the exposure condition. In practice, the threshold dose is multiplied by a reduction factor to determine an MPE. Consensus is required to deem that the MPE for the given exposure conditions does not produce an adverse biological response. After discussions, reviews, and votes, exposure guidelines published. Although complex, MPEs are set at levels that are both protective but do not restrict safe, beneficial applications. Hence, elements of risk assessment have intrinsically been built into MPE or exposure limits.
While the MPEs are important to determine relative risk, effective laser safety standards must include practical guidance to promote and assure safe practices. Classification and control measures are integral and critical to those practices. Control measures are detailed in Section 4 and are dependent upon the system classification. For Classification of lasers or laser systems, the accessible laser or laser system emission is given in terms of the Accessible Emission Limit (AEL) expressed in watts for continuous wave lasers and in joules for pulsed laser systems. The AEL is a readily measureable quantity provided the measurement aperture is defined for the exposure condition. For example, the AEL for a visible laser operating in the retinal hazard region is determined by multiplying the MPE given in terms or the corneal radiant exposure or corneal irradiance by the area defined by a 7 mm measurement ( a large, near “worst case” pupillary) aperture. Measurement apertures for the purpose of classification are given in Table 9 in ANSI Z136.1-2014 (Ref 1) for other exposure conditions.
While some consider laser classification to be complex, the guidelines accommodate many diverse situations with complex inclusion of relative risk. When the Class is determined, the simplified ordinal Class scale is a rapid indicator or relative risk. Several examples of laser or laser system classification are given in Appendix B of ANSI Z136.1-2014 (Ref 1). Section B4 specifically addresses Laser Classification and provides instructive classification examples. Section B4.1 provides classification examples for unaided viewing (Condition 2) and Section B4.2 for all viewing conditions that includes optical aided viewing (Condition 1). Example 20 in Section B4.1 explores the classification of an argon laser emitting 1 watt. As the example illustrates, classification is dependent on the configuration of the system and could range from Class 1 if the beam is totally enclosed in a “sealed pipe” or Class4 if the accessible beam exceeds 0.5 watts in a small beam. Instructive classification examples of more complex systems (e.g. repetitive pulsed systems) included in Section B4 are indicative of the inherent complexity of Classification. However, these examples also indicate the importance of Classification to indicate the actual hazard or risk based upon actual configuration or use.
Classification determines the nature and number of control measures required. Control measures required for a Class 3B system far exceed those required for a Class 2 laser but are not as stringent as required for a Class 4 laser. Tables 10 and 11 in ANSI Z136.1-2014 (Ref 1) provide a helpful list of control measures required as a function of Class.
The recently updated entitled Laser Safety Guide (Ref 5) booklet published by the Laser Institute of America r summarizes the degree of hazard or risk for the seven laser classes in accordance with the 2014 guidance and exposure limits provided in ANSI Z136.1-2014 (Ref 1). A verbatim excerpt from this booklet is included as Appendix A to emphasize the relationship between laser or laser system class and risk.
Summary and Recommendation
Laser and laser system classification indicates the relative hazard or risk of a system implementation. The laser or laser system Class is based upon the emission characteristics and the use circumstances. The Class is anchored to selected maximum permissible exposures (MPEs). Despite the complexity of the classification process, the laser class provides the user with an immediate assessment of relative risk in an ordinal scale (Class 1 to Class 4). Recommend ANSI Z136.1-2014 be consulted for comprehensive classification details. If further assistance in Classification is required, recommend a laser safety specialist or Certified Laser Safety Officer by consulted.
The following extracted from Ref 5 provides a description of the relative risk of each class of laser or laser system.
Class 1 denotes lasers or laser systems that do not, under normal operating conditions, pose a hazard. Class 1 also includes those lasers that were previously designated as Class 2a.
Class 1M denotes Class 1 lasers or laser systems that do not normally pose a hazard for unaided viewing unless the beam is viewed with collecting optics, e.g., telescope.
Class 2 denotes low power visible lasers or laser systems which, because of the normal human bright-light aversion response (i.e. blinking, eye movement, etc.), do not normally present a hazard, but may present some potential for hazard if viewed directly for extended periods (like many conventional light sources). The signal word “CAUTION” is used with all signs and labels associated with Class 2, Class 2M and some Class 3R lasers warning against staring into the beam (see Fig. 7).
Class 2M denotes Class 2 lasers or laser systems that do not normally pose a hazard for unaided viewing; however, are potentially hazardous if viewed with collecting optics.
Class 3R denotes lasers or laser systems that have a lower risk of producing eye injury than other Class 3 lasers, and therefore have reduced requirements from those lasers that have a higher associated risk. Most lasers or laser systems previously classified as Class 3a can be treated as Class 3R including many laser pointers.
Note: Products can be classified as Class 1M and Class 2M even if their output exceeds the Class 3R level as long as the output does not exceed the Class 3B level.
Class 3B denotes lasers or laser systems that can produce a hazard if viewed directly, including intrabeam viewing of specular reflections. Normally, Class 3B lasers will not produce a hazardous diffuse reflection from a matte (not shiny) target. The signal word “WARNING” (Fig. 8) is required for Class 3B and most Class 4 lasers and laser systems.
Class 4 denotes lasers or laser systems that produce a hazard from not only direct or specular reflections, but may also produce hazardous diffuse reflections. Such lasers may produce significant skin hazards as well as fire hazards. The signal word “DANGER” (Fig. 9) is restricted to those Class 4 lasers with high (i.e., multi-kilowatt) output power or pulse energies with exposed beams.
1. ANSI Z136.1-2014, American National Standard for the Safe Use of Lasers, Orlando, FL: Laser Institute of America (2014).
2. Wolbarsht ML and Sliney DH. Safety with Lasers and Other Optical Sources. New York: Plenum Publishing Corp (1980).
3. Henderson R and Schulmeister K. Laser Safety. Bristol and Philadelphia: Institute of Physics Publishing. (2004). ISBN 0-7503-0859-1
4. Lund DJ. The New Maximum Permissible Exposure: A Biophysical Basis. IN: Laser Safety: Tools and Training. K. Barat, Ed., Boca Raton: CRC Press, pp145-175 (2014).
5. Laser Institute of America. Laser Safety Guide. DH Sliney, Ed., Orlando: Laser Institute of America, pp9-10 (2015). ISBN 978-1-940168-03-6