MPE and NHZ Calculation Guide

Calculating MPE and NHZ

The Laser MPE and NHZ Calculations document provides the laser user with formula’s to perform the MPE and NHZ calculations for lasers and laser systems used at the University of Chicago. The results from these calculations will assist the user in establishing the engineering and administrative controls for each laser and laser system used in their laboratory.

The Maximum Permissible Exposure (MPE)

The single most useful number in laser safety calculations is the Maximum Permissible Exposure (MPE). This is the minimum irradiance or radiant exposure that may be incident upon the eye (or the skin) without causing biological damage.

The Nominal Hazard Zone (NHZ)

The other major definition for laser safety calculations is the Nominal Hazard Zone (NHZ). This is a distance within which the irradiance of a beam is greater than the MPE. Besides being specific to a given wavelength and time of exposure, a different NHZ can be defined for the beam’s path to your eye – direct viewing, specular reflectance, or diffuse reflectance.

The NHZ is a practical definition; it has a specific shape around your particular laboratory apparatus (for instance, assuming your lab has no windows and a solid door, the NHZ will in the worst case scenario be the floor area of the lab itself). In other words, the NHZ for you will be derived at the end of your safety calculations, and thereafter will be most useful to you for planning control measures in you laboratory. Formulas for calculating the NHZ are listed below.