Hazard Awareness - Lasers

Laser Safety

UPDATE: Engineering Physics recently acquired copies of the ANSI Laser Safety Standard Z136.1-2007. Copies can be signed out for loan from the department office.

The CEDT and Engineering Physics use lasers in a variety of applications, including photoluminescence, ellipsometry, grating fabrication, bench-top photonics experiments, and thin film stress analysis.

McMaster University's laser safety program is documented in Risk Management Manual program RMM 703.

Potential Hazards

The most significant laser risk stems from the absorption of laser radiation in tissue. If tissue is unable to effectively dissipate the resultant heat it can be burned and permanently damaged. Naturally, the eye is most susceptible to this type of damage since it can focus light onto the retina, increasing the intensity significantly. Retinal burning can occur essentially instantaneously and may even be accompanied by explosive localised gaseous emission.

A less serious, though still considerable, risk associated with lasers is that of photochemical reaction stimulated by short-wavelength radiation.

Additional risks in laser operation include electrical shock, burns, generation of toxic air pollutants, and fire, among others.

Hazard Classification

The American National Standards Institute (ANSI) established the widely accepted the ANSI Z136.1 standard for laser safety in 1976.* The standard identified maximum permissible exposure (MPE) limits that are considered safe for laser operators.

ANSI Z136.1 also established a classification system that groups lasers into four classes depending on the power of the laser and the wavelength of emitted light. Table 1 summarizes the ANSI z136.1 class system. (For pulsed lasers, please refer to Mallow and Chabot [1978], or Sliney and Worbarsht [1980]).

*The Z136.1 standard was originally released in 1973, but was revised in 1976. A class 5 designation was dropped, and class 1 emission limits were changed [Sliney, 1980]. McMaster is currently implementing the standards outlined in the most recent revision, ANSI Z136 2007.

Laser Safety and Maximum Permissible Exposure

Any laser experiment should incorporate engineering protocols to ensure the safety of laser operators. That is, wherever possible laser beams should be enclosed to prevent accidental exposure. This is the most effective protective measure available when using class 3b or class 4 lasers.

Unfortunately, many experimental arrangements require direct operator interaction, particularly in the alignment of laser beams. Obviously, in this situation protective enclosures become ineffective and therefore "personal protective measures" become absolutely critical. A laser operator working with class 3b or 4 lasers (or some class 3a lasers when optical instruments may increase beam intensity) must use protective eyewear. The eyewear must be suitable to reduce the intensity of a beam below the MPE for the type of laser radiation in question.

Determining the MPE for a specific laser is non-trivial. Relevant factors include pupil size, wavelength, exposure duration, size of irradiated area (intrabeam or extended source viewing), incident irradiance, and continuous or pulsed laser output.

Essentially, the MPE determines the minimum attenuating strength required for eye protection to be considered safe. That is, eye protection must be chosen based on the MPE for a given laser such that an accidental exposure is rendered eye-safe, similar to that of a class 3a or lesser laser. Optical density (OD) quantifies the ability of a filter to attenuate a particular wavelength according to the formula,

OD = log_10 [Io/I]

where Io is the irradiance [W/cm^2] of the incident beam and I is the irradiance of the transmitted beam.

For example, a frequency doubled Nd:YAG laser emitting 300mW at 532nm that is attenuated by a OD 6 filter will transmit 0.0003mW. Attenuated by an OD 1.5 filter, the same laser will still transmit 9.5mW.

Please note that OD is wavelength-specific. Since many lasers emit at more than one wavelength, the OD of a filter or set of filters must take into consideration each wavelength present in the beam.

Though rigorous calculations of MPE can be worthwhile, a number of sources present a simplified guide to selecting laser eye protection, as originally developed by the U.S. Army Environmental Hygiene Agency.

Choosing Protective Eyewear

To choose protective eyewear suitable for use with a particular continuous wave (CW) laser where there exists risk of momentary eye exposure:

1. Determine the wavelength or wavelengths of the laser (usually labelled on commercial lasers).
2. Determine the power (W) or irradiance (W.cm^-2) for the laser (usually labelled on commercial lasers).
3. Locate the corresponding power or irradiance range in columns 5 or 6 of Table 2.
4. Identify the corresponding optical density listed in column 10 of Table 2.
5. Obtain appropriate protective eyewear.

Again, please note that some lasers may emit radiation at more than one wavelength. Glasses suitable for a laser's primary wavelength may offer no protection at another wavelength, though the power emitted at the secondary wavelength may still be sufficient to damage the eye. All wavelengths must be considered and appropriate eyewear procured.