F/X On-line Newsletter - Special Reports
Audience Scanning Handout
Laser Safety Seminar
November 8, 1998 ·
Amsterdam, The Netherlands
Moderator: Tony Zmorenski, Safety Officer, Walt Disney
Geoff Jones, Entertainment Laser Association, UK
Dr. Daniel Clark, Loughborough College, UK
John O'Hagan, National Radiological Protection Board, UK
Dirk Baur, MediaLas GmbH, Germany
Dr. Wolfgang Kirchner, WKL, Germany
Greg Makhov, ILDA Safety Committee
Patrick Murphy, Pangolin Laser Systems, USA
Bill Benner, Pangolin Laser Systems, USA
Lasers can be eye-safe, up to a certain level.
Beyond a certain level, lasers are definitely eye hazards
There must be safe procedures to prevent damage
But first we must know: At what light level does there begin to be damage?
The Concept of MPE Levels
Since 1972, there has been an internationally agreed-upon
hazard concept: the "MPE" or Maximum Permissible Exposure.
Actually, the "MPE" includes many different exposure limits,
depending on the laser wavelength and time of exposure.
The MPE for a given wavelength and exposure duration means: 10 times less
than the light level where 50% of subjects' eyes had visible damage.
Expressed another way: shining light at the MPE level into a subject's eye
has a statistical chance of damaging 3 out of every 100 subjects.
In summary, the MPE is a worst-case "safety factor". Exposure at
the MPE level is already somewhat hazardous (statistically 3 out of 100
eyes would show signs of visible damage.) So additional factors such as a
moving beam are assumed to further reduce the risk.
International Laws and Regulations
The MPE levels were derived from studies and discussions
by international experts in laser safety. The MPE is here to stay. It
would be difficult or impossible to try to change the MPE.
Public safety is expressed by standards which set the maximum legal
These standards set forth essentially the same MPE in most industrialized
nations. There are not "different standards in different
What differs is enforcement:
-- Some countries are very strict, such as the United States. They demand
extensive proof before they allow audience scanning.
-- Some countries were lenient, but now are strict such as the United
Kingdom. Owing to the efforts of just two people, audience scanning
enforcement was tightened significantly.
-- Some countries are lenient or may not understand or apply the
standards. In these countries, it is routinely possible to scan audiences
with light significantly exceeding the MPEs.
Responsibilities of Laser Operators
If in a country with strict enforcement, the regulations
must be followed.
If in a country without strict enforcement, the MPE levels must be
followed, for three reasons:
-- Companies must be professional. It is unprofessional and risky to
produce an unsafe show (or a show where the hazards are not analyzed).
-- Companies must be proactive. This is to avoid government regulations,
or clients worried by press reports (e.g., laser pointers)
-- Companies must be aware of what will happen if only a few well-placed
people start to examine shows, and find unsafe ones (e.g., U.K.)
Safety can be a marketing advantage against low-budget, unsafe
competitors. Do clients want to risk harm to their audience?
Technical Factors in Audience Scanning
Non-scanning effects (audience illumination): Diffraction
gratings, lumia light washes.
High-inertia scanners (takes >1 second to stop): Mirror balls, cone
scanner, polygon scanner.
-- Relatively safe -- easy to detect failure, time to close shutters
-- These have been approved in the U.S. for audience scanning.
Low-inertia scanners (takes less than 1 second to stop): Galvanometer
scanners, AO deflectors.
Riskier -- harder to detect failure, shorter time to stop light output
-- Needs a very fast shutter such as a PCAOM.
-- Unapproved (or very rare) in the U.S. for audience scanning
(Note: The rest of the discussion focuses on low-inertia scanners;
specifically, galvo scanners)
Measurements of the MPE Levels
There are MPEs for three types of exposure: average,
single pulse, reduced single pulse.
Because we are talking about scanning, in most situations only
"reduced single pulse" needs to be examined. This is more than
one pulse (scan) in 1/4 second.
Even a fan scan (scanning a line and moving it down the audience) would
cause multiple pulses in less than 1/4 second, into a single eye. Thus,
"reduced single pulse" applies.
The pulse duration (how long the beam scans across a 7mm aperture) is the
most important measurement.
Measuring with an oscilloscope and detector.
Measuring with a LOBO LMS-2 meter
Measuring with a POE MPE Meter.
Audience Scanning Safety Practices (see below for more details)
Do not scan with pulsed lasers
Don't use a single beam
Move the projected effects
Don't rely on faster scanning
Attenuate power with size
Limit anchor (dwell) points
Scan fail interlock
Program "no-exposure" conditions in the show
Measure the irradiance
Quick test for aversion response
Respect the audience
Detailed Discussion and Techniques
Discussion about MPEs and international safety standards
ELA training programme
Using a scope, LMS-2 and MPE Meter
The "10-second" rule for show analysis
Scanning Safety Practices
No system or test can absolutely guarantee eye safety when
deliberately scanning the audience. You should use accepted instruments and
practices to check the questionable parts of your show. The following tips
are general ways to make your show safer through good design practices, and
if accepted instruments are not available at your show site.
Do Not Scan with
Pulsed lasers (e.g., metal vapour, pulsed YAG, pulsed
solid-state) are inherently hazardous due to the power of each pulse. It
requires exacting calculations to even consider scanning an audience with
pulsed lasers. Because of the great potential danger, use continuous wave
lasers (e.g., HeNe, argon, krypton, diodes, CW YAG, CW solid-state) only.
For ranges of less than 30 meters, using a lens to increase
divergence can allow for visually effective power levels while maintaining
controllable irradiance levels. A bright, fuzzy beam is far safer (and more
visually effective) than a dim, tight beam with the same irradiance.
Don't Use a Single
You should never aim a single beam into the audience. In general,
if a single beam is safe, then any scanned effects such as cones and fans
will spread the light out, and be too dim to be effective.
Move the Projected
When projecting a fan or tunnel, move the effect through the
audience. This reduces the multiple pulse accumulation.
Don't Rely on Faster
In general, you will not increase safety by scanning faster.
Although the beam spends less time in the eye, there are more crossings of
the eye, and thus the total light energy delivered remains about the same.
Attenuate Power with
The smaller the projected effect, the greater the concentration of
energy. Any effect that grows from a point, or shrinks to a point should
have a proportional fade in/ fade out.
Limit Anchor (Dwell)
Anchor points reduce beam velocity and increase exposure. Where
possible, use blanking to emphasise beams, rather than anchor points.
Scan Fail Interlock
Use a scan fail interlock of some sort. Chances of a still beam
from a laser entering someone's eye are small, but consider the
"No-Exposure" Conditions in the Show
Allow time for the eyes to recover by parking effects outside of
the audience area. A good "no-exposure" time is 10 seconds or
Typical shows should not exceed 10 milliwatts per centimeter
squared, or 100 watts per meter squared. You need to know what you are
delivering to the audience.
Quick Test for
Note: The following tip is only for use when you believe your show
is safe by using the above tips AND you are aware you could damage your eyes
if your show is not safe. Use your computer or PCAOM controls to set the
laser output to all green or all white. Run the show while standing at the
closest audience access point. As the laser crosses your eyes, evaluate the
brightness. If you have a desire to avert your eyes, you are probably
approaching or exceeding the internationally agreed safety levels (MPE).
Respect the Audience
Not everyone enjoys bright lights in their eyes. Remember that they
trust you to ensure their safety.
BACK to Afternoon Session
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this special report down into a number of pages to speed access:
SPECIAL REPORT -
Introduction and Background
6 Nov - PUG MEETING
7 Nov - ATW (Advanced
7 Nov - Canal ride and Dinner
(NOTE: Large page long download time)
8 Nov - Meeting, Seminars and
9 Nov - Seminars and Awards
ILDA Trade Show - A
(NOTE: Large page long download time)
ILDA Trade Show - B
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Candid Camera (NOTE:
Large page long download time)
Essay: Sex and Drugs in Amsterdam
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