The Nominal Hazard Zone
The nominal hazard zone (NHZ) associated with Class IIIB and Class IV lasers
shall also be determined. The NHZ describes the space within which the level of
direct, reflected, or scattered radiation during normal operation exceeds the
appropriate MPEs and is determined from the following characteristics of the
laser:
- Power or energy output.
- Beam diameter.
- Beam divergence.
- Pulse repetition frequency (prf).
- Wavelength.
- Beam path including reflections.
- Beam profile.
- Maximum anticipated exposure duration
Examples of NHZ calculations are given in the appendix of ANSI Z136.1 (1986).
In addition, computer software is also available to assist in the computations
for NHZ, protective eyewear optical densities and other aspects of laser hazard
analysis.
It is often necessary in some applications where open beams are required (e.g.,
industrial processing, laser robotics) to define the area where the possibility
exists for potentially hazardous exposure. This is done by determining the Nominal
Hazard Zone (NHZ) which is, by definition, described by the space within which
the level of direct, reflected or scattered radiation exceeds the level of the
applicable MPE. Consequently, persons outside the NHZ boundary would be exposed
below the MPE level and are considered to be in a "safe" location. The NHZ boundary
may be defined by direct (intrabeam) beams, diffusely scattered laser beams as-well-as
beams transmitted from fiber optics and/or through lens trains, etc. In other
words, the NHZ perimeter is the envelope of MPE exposure levels from any specific
laser installation geometry.
The purpose of an NHZ evaluation is to define that region where control measures
are required. Thus, as the scope of laser uses has expanded, the classic method
of controlling lasers by enclosing them in an interlocked room has become limiting
and, in many instances, can be an expensive over-reaction to the real hazards
present.
