Understanding laser safety classes.

The level of laser exposure which is considered as the limit between safe and potentially harmful is called Maximum Permissible Exposure (or MPE). Maximum Permissible Exposures are set by the International Commission on Non-Ionizing Radiation Protection (ICNIRP) and are also adopted by standardisation committees.

As Maximum Permissible Exposure evaluation and the determination of hazard areas (NHZ: Nominal Hazard Zone) are quite involved, a laser safety classification scheme has been designed by international standardisation committees to help users to decide if their laser is a potential hazard. Below is a summary of the different laser classes with their description.

Class 1

  • Meaning: safe
  • Type of laser: very low power lasers or enclosed lasers.
  • Maximum Permissible Exposure: is never exceeded, even for very long exposure (hours), or with the use of optical instruments.
  • Nominal Hazard Zone: none.
  • Typical Accessible Emission Limit*: 40 µW for blue.

Class 1M

  • Meaning: safe for the naked eye only, but potentially hazardous when optical instruments** are used.
  • Type of laser: medium power lasers either collimated with a large beam or highly divergent.
  • Maximum Permissible Exposure: can be exceeded when using optical instruments**.
  • Nominal Hazard Zone: none for the naked eye.
  • Typical Accessible Emission Limit*: a laser can be classified as Class 1M if the total output power is below class 3B (0.5 W for continuous in the visible) but the power that can pass through the pupil of the eye is within Class 1.

Class 2

  • Meaning: safe for unintended exposure, (less than 0.25 s) but hazardous when looking at for more than 0.25 s.
  • Type of laser: visible (400–700 nm) low power lasers.
  • Maximum Permissible Exposure: are not exceeded provided the viewings areaccidental only. MPE calculation assumes the blink reflex will stop the light after 0.25 s
  • Nominal Hazard Zone: none for accidental exposure.
  • Typical Accessible Emission Limit*: 1 mW for continuous lasers.

Class 2M

  • Meaning: safe for the naked eye when the exposure is unintended, (less than 0.25 s) but hazardous when looking at for more than 0.25 s or when optical instruments** are used.
  • Type of laser: visible (400–700 nm) medium power lasers either collimated with a large beam or highly divergent.
  • Maximum Permissible Exposure: are not exceeded provided the viewings areaccidental only and only with naked eyes. MPE calculation assumes the blink reflex will stop the light after 0.25 s. Using optical instruments** might bring the exposure above the MPE as well.
  • Nominal Hazard Zone: none for accidental exposure to the naked eye.
  • Typical Accessible Emission Limit*: a laser can be classified as Class 2M if the total output power is below class 3B (0.5 W for continuous in the visible) but the power that can pass through the pupil of the eye is within Class 2.

Class 3R

  • Meaning: unsafe, except when handled carefully by experienced users. Accidental short exposure is considered as a small hazard.
  • Type of laser: low power lasers.
  • Maximum Permissible Exposure: can be exceeded up to 5 times.
  • Nominal Hazard Zone: hazard area for the eye, none for the skin.
  • Typical Accessible Emission Limit*: typically 5 mW in the visible.

Class 3B

  • Meaning: unsafe without exception, Personal Protective Equipment (laser safety goggle) must be worn within the nominal hazard zone. Focused lasers of this class are a potential fire hazard.
  • Type of laser: medium power lasers.
  • Maximum Permissible Exposure: is exceeded more than 5 times. Skin MPE is not generally exceeded, except at focus.
  • Nominal Hazard Zone: hazard area for the eye, none for the skin.
  • Typical Accessible Emission Limit*: 500 mW.

Class 4

  • Meaning: dangerous, Personal Protective Equipment for eyes and skin must be worn within the nominal hazard zone. Class 4 lasers are fire hazards as well. Diffuse reflections may be hazardous. Those lasers are commonly used for cutting or welding. This can create hazardous fumes. Cutting lasers generally create a small plasma which in turn emits UV light. UV light is another hazard to consider on a manufacturing floor.
  • Type of laser: high power lasers.
  • Maximum Permissible Exposure: ocular and skin MPE are exceeded. Diffuse reflections exceed the Minimal Permissible Exposure.
  • Nominal Hazard Zone: hazard area for the eye and for the skin.
  • Typical Accessible Emission Limit*: no limit.

NotesAccessible Emission Limit (AEL): an AEL is the maximum value of accessible laser radiation to which an individual could be exposed during the operation of a laser and is dependent on the laser class. The AEL above are given as an indication for continuous lasers, but may change for pulsed lasers or infrared lasers.

Optical instruments: two types of optical instruments increase the hazard of M lasers:

  • instruments which will reduce the diameter of a collimated beam (telescopes, beam reducers, binoculars). This is dangerous when using lasers with large beams (>7mm) since it is likely to increase the amount of light entering the pupil of the eye.
  • Converging optics such as lenses, loupes, prescription eyewear… this is an increased hazard when using highly divergent beams since it will make it less divergent for the eye, allowing a greater amount of light to enter the eye.

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