Audience Scanning Calculations
In making safety calculations for audience scanning, there are two steps. The first is to determine the static laser beam parameters, such as laser power, divergence, audience distance etc. The result will give the irradiance, MPE, Nominal Ocular Hazard Distance (NOHD), and other safety-related characteristics for a fixed beam at the closest point of audience access. This is the "worst case".
The second step is to calculate the effect of scanning this fixed beam. As the beam passes one or more times over the eye, it creates one or more pulses. Thus, single- and multiple-pulse MPEs come into play. For simple, repeated scan patterns, it is possible to calculate the maximum possible exposure, by looking at the location (such as an edge) where the beam is moving slowest (longest dwell time).
But for a laser show, where there are many different scan patterns, it is nearly impossible to calculate the "worst case" location for viewing the show. This is where making multiple measurements of the show can help.
Doing measurements requires caution in using the proper type of instruments, and in correctly setting up and interpreting the measurements. There is no single right answer, since a detector could "pass" the show in one location but "fail" it in another. Taking measurements at a number of different locations—say 10 or 20—can give a general indication as to the show's intensity. However this also requires running the show 10 or 20 times, which can be difficult in many applications (e.g., a one-time, one-night show).
There are a few commercial systems available which help with audience scanning calculations and measurements. One software program is available in a free "Lite" version which can be used to determine basic beam parameters, and worst-case static beam and single pulse calculations.
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