title: Green laser pointers for visual astronomy: how much power is enough?
abstract: OBJECTIVE
  Green laser pointers with output powers in the tens to hundreds of milliwatt (mW) range, clearly exceeding the limiting 5 mW of American National Standards Institute class 3a (International Electrotechnical Commission class 3R), are now easily available in the global market. They are increasingly being used in public sky observations and other nighttime outreach activities by educators and science communicators in countries where their use is not well regulated, despite the fact that such high power levels may represent a potential threat to visual health. The purpose of this study was to determine the output power reasonably required to perform satisfactorily this kind of activities.
  
  
  METHODS
  Twenty-three observers were asked to vary continuously the output power of a green laser source (wavelength 532 nm) until clearly seeing the laser beam propagating skyward through the atmosphere in a heavily light-polluted urban setting. Measurements were conducted with observers of a wide range of ages (9 to 56 years), refractions (spherical equivalents -8.50 to +1.50 diopters), and previous expertise in using lasers as pointing devices outdoors (from no experience to professional astronomers). Two measurement runs were made in different nights under different meteorological conditions.
  
  
  RESULTS
  The output power chosen by observers in the first run (11 observers) averaged to 1.84 mW (+/-0.68 mW, 1 SD). The second run (17 observers) averaged to 2.91 mW (+/-1.54 mW). The global average was 2.38 mW (+/-1.30 mW). Only one observer scored 5.6 mW, just above the class 3a limit. The power chosen by the remaining 22 observers ranged from 1.37 to 3.53 mW.
  
  
  CONCLUSIONS
  Green laser pointers with output powers below 5 mW (laser classes American National Standards Institute 3a or International Electrotechnical Commission 3R) appear to be sufficient for use in educational nighttime outdoors activities, providing enough bright beams at reasonable safety levels.
authors.affiliation: Area de Optica, Dept. Física Aplicada, Optics and Optometry School, Universidade de Santiago de Compostela, Galiza, Spain. salva.bara@usc.es
authors.forename: Salvador
authors.initials: S
authors.lastname: Bará
authors.forename: Marisol
authors.initials: M
authors.lastname: Robles
authors.forename: Isabel
authors.initials: I
authors.lastname: Tejelo
authors.forename: Ramón I
authors.initials: RI
authors.lastname: Marzoa
authors.forename: Héctor
authors.initials: H
authors.lastname: González
mesh_terms: D000293:Adolescent; D000328:Adult; D001256:Astronomy / Q000295:instrumentation*; D002648:Child; D003116:Color; D004307:Dose-Response Relationship, Radiation; D006801:Humans; D007834:Lasers; D008875:Middle Aged; D012449:Safety; D055815:Young Adult