Maximum permissible exposure (MPE)
The single most useful number in laser safety calculations is the maximum permissible exposure. This is the minimum irradiance or radiant exposure that may be incident upon the eye or skin without causing biological damage. The MPE varies by wavelength and duration of exposure and is documented in tables published in ANSI z136.1 standard. Think of the MPE as your laser safety speed limit.
Nominal Ocular Hazard Distance (NOHD)
The Nominal Ocular Hazard Distance (NOHD), sometimes referred to as the Nominal Hazard Distance, is the distance along the axis of the emitted beam at which the irradiance is equal to the MPE. The NOHD is dependent on beam characteristics such as the power, diameter, and divergence. The NOHD is usually much greater than the largest dimension of your laboratory space.
Nominal Hazard Zone (NHZ)
The Nominal Hazard Zone (NHZ). This is a distance within which exposure to a direct, reflected, or scattered beam is greater than the MPE. Mirrors, optics, and reflective materials in the beam path may result in diffuse or specular reflections in unintended directions. Specular reflections are hazardous over a greater range than diffuse reflections. If you are in the NHZ, you are at risk of an exposure above the MPE.
Laser Safety Calculations: Calculating MPE, NOHD, and NHZ
The Laser Safety Calculation Guide (PDF) has been developed to provide an overview of how to perform the MPE, NOHD and NHZ calculations for lasers and laser systems used at the University of Chicago to assist the user in laser hazard evaluations. Important definitions and a summary of the most common causes of laser accidents are added to provide the laser user more context. Additional information and resources are available through the Laser Safety Officer.