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Making shows safe and enjoyable
By William R. Benner,
Jr.
(An earlier version of this article originally
appeared in the fall 1997 edition of The Laserist Magazine.)
Successfully determining the safety level of
an Audience Scanning show requires not only the proper tools, but also an understanding
of the theory behind safety exposure limits and the ability to
correctly interpret measurement data. This article is intended to
explain the basic concepts of Audience Scanning evaluation, along
with a "hands on" approach to evaluating audience scanning effects
within a laser show.
This article is based on a lot of research into audience scanning safety,
and has also been reviewed by safety experts including Greg
Makhov, ILDA Safety Committee Chairman, and John O'Hagan
of the UK National Radiological Protection Board. Since the
resources used in preparing this article have been mainly based in
the United States and the United Kingdom, there is a possibility that in other countries, different
analysis techniques can be used to evaluate beams projected into the
audience. Moreover, in some countries such as Sweden, it is illegal
to scan the audience with a laser beam. Because of this, you should
seek the advice of local regulatory officials.
Before explaining how to evaluate a
show, it might be good to answer a very fundamental question -- why
go to the trouble of evaluating a show to make sure that it is safe?
There are at least three answers to this question:
- To avoid possible legal action
If someone is exposed to an unsafe show, it is possible the show
will harm his or her vision. In climates of heavy litigation such
as the United States, it is likely that this person would seek
legal action against the venue owner and the show producer. If the
show was determined to be operating at unsafe levels, it raises
the probability of that person being awarded a claim of damages,
even if the harm was not caused by the show. If on the other hand,
shows were being done completely safely, no vision damage would
occur. Even if someone decided to try frivolous legal action, it is
unlikely that they would be able to collect a claim if the show is
operating at safe levels.
- Safe audience scanning shows are more enjoyable than unsafe
ones
When operated at safe levels, audience scanning shows leave
little or no afterimages, which makes the whole show enjoyable.
You should test this yourself. The next time you have the
opportunity to view audience scanning shows, pay particularly
close attention to your vision as various effects cross your eyes.
Effects that appear to leave a strong afterimage are not pleasant
to experience and detract from the whole show. When this happens,
your eyes will be too busy recovering from the last effect to
enjoy the next one. However, effects that leave little or no
afterimage are beautiful and fun to experience.
- Because it can be done safely
As humans, if something seems difficult to achieve, we
sometimes would rather avoid it instead of challenging it. As you
start to learn about the many aspects of laser safety, it may seem
as though there are far too many variables and formulas to
calculate, which makes audience scanning safety hard to evaluate.
Instead of seeking the help of safety experts, it is easier to
simply deny that there is a safety problem. This article will
first introduce some definitions of terminology, and then show you
how to evaluate an audience scanning show.
Definitions of Terminology
- Irradiance
Probably the most important and fundamental concept to laser
safety is irradiance. This is a big word that simply means
concentration of laser power per unit area. It can be found by
dividing the power of the beam by the area that covers. For
example, if a 1 watt beam covers a 1 square centimeter area, it
has an irradiance of 1 watt per square centimeter. If that beam is
allowed to diverge to cover a 2 square centimeter area, notice
that it has expanded in both width and height. In this case the
irradiance is 1 watt / 4cm (2cm high X 2cm wide) = 0.25 watts per
square centimeter. Although the irradiance has greatly decreased
because the beam has expanded to cover a larger area, the total
beam power is still exactly 1 watt.
The reason that irradiance is so
pivotal to laser safety is that for safety evaluation, your eye
is considered to have a pupil diameter of 7mm. Any beam that
enters your eye with a diameter of 7mm or smaller, will deliver
the full power of that beam into your eye. However, if the beam is
larger than 7mm, your eye will only be exposed to the portion of
the beam which is allowed to enter the eye.
The ANSI Z136.1 Standard for Safe Use
of Lasers uses watts per square centimeter as the unit of measure
for safety evaluation. Some other standards use watts per square
meter as the unit of measure.
- Average power versus Peak power
Lets say that you want to project the image of a circle onto a
screen. There are at least two ways of producing that image: 1)
directing a beam through an optical element such as a diffraction
grating; 2) directing a beam to a set of scanners which can
rapidly draw the circle and make it appear solid. The reason that
scanners can create the same solid image on a screen is because of
a phenomenon called persistence of vision.
If the diffraction grating and the scanners produce the
same image, they will have the same brightness on the screen
because they will be spreading the total laser power out around
the screen by the same amount. At any one point along the circle,
the average power may be 1000 times less than the power of the
laser.
However, just because these images look the same, don't
forget that in the case of the scanners, there is just one spot on
the screen at any one point in time, and this spot has the entire
power of the laser. If you place your eye at that point along the
circle, and the beam diameter is 7mm or less, your eye will
receive the total power of that laser each time the beam scans
past your pupil. Using our 1 watt laser as an example, even though
the average power is 1mW, the peak power that your eye will get is
1 watt! This peak power is the most hazardous and easiest
overlooked element of audience scanning safety evaluation.
- Pulses and Multiple pulses
When a laser beam scans across the pupil, it is said to deliver a
pulse of laser light to your eye. This is because as the beam
scans past your eye, it will only enter your eye for a brief time
depending on the beam diameter and the scan rate. This pulse of
light created by the scanned beam is similar to a pulse that is
created by a beam that is not scanning, but is turned on for only
a brief instant. The amount of time that the beam is on within
your pupil is called the pulse-width. For audience scanning
shows, this pulse-width is commonly 20 to 500 microseconds.
When you project an effect such as a tunnel or sheet scan,
this is done by continually scanning the tunnel or sheet to make
them appear solid. As the beam crosses your eye, it will allow a
pulse of light to enter your eye. Since the scanners will trace
this effect many times to make it appear to be solid, your eye
will receive multiple pulses of light.
The reason that the concept of pulses
and multiple pulses is important is because safety standards
prescribe a maximum amount of light that you can be exposed to for
a single pulse, and for multiple pulses.
How to evaluate an audience scanning show
Within this article we will discuss how to evaluate a show using
manual calculations assisted by basic measurement tools. This
requires the
ability to project a stationary effect. Although your laser
projector can be scanning an effect such as a sheet scan, tunnel or
array of beams, the effect itself must remain stationary because a
light
detector has to be placed into the scanned beam effect to obtain an accurate
measurement. For this reason, ADAT or other taped shows cannot be
effectively evaluated using these techniques.
Tools Needed for Manual Calculations Assisted by Basic
Measurements
- Calibrated Laser Power Meter
You must use a laser power meter designed to measure static
(non-moving and non-modulated) beams. Since the meter should be
able to measure low light levels, the meter should use a silicon detector
with flat spectral response. For ease in performing safety evaluations,
you should use one with an active area of 1 cm2 (one
square centimeter). Using this size detector is easier because if
the beam fills or overfills this detector, the meter will be
extraneously measuring irradiance (concentration of laser power)
in watts per square centimeter. Although other detector sizes can
be used, you would have to perform a calculation to convert units
of measure. For simplicity, this article assumes that a 1cm2
detector will be used.
- Fast Silicon Photodiode with Amplifier
Fast silicon photodiodes are available from several vendors,
including Hamamatsu, Centronic and UDT. Since these devices output
a current instead of a voltage, an external amplifier must be used
to facilitate connecting them to a scope. Alternatively, you can
purchase a detector with a built-in amplifier such as the OSI
series from Centronic. For audience scanning pulse-width
measurement, the active area of the detector should be 7mm or
greater. If it is greater than 7mm, you will need to make a mask
with a 7mm hole and place it over the detector. This is called a
limiting aperture. (7mm is the internationally recognized ocular
pupil diameter to be used for safety evaluation.)
- Oscilloscope
An oscilloscope will be used along with the fast silicon
photodiode to measure the pulse-width and pulse repetition rate.
Analog field scopes with a vertical bandwidth of 50MHz or greater
will work fine. Digitizing oscilloscopes should be used with
caution because sample aliasing can result if you are not careful.
- Scientific calculator
Any calculator capable of doing exponents and powers of ten will
work fine. Often, I use the Calculator program that comes with
Microsoft Windows, (select "scientific" mode from the view menu).
- Some technical skill...
Manual audience scanning safety evaluation is quite tedious and
error prone. It requires knowledge and experience to use the tools
specified above and should only be done by individuals who are
technically adept.
Evaluating the show
After the equipment has been prepared, you should run the entire
show several times to identify and list particularly bright and
hazardous effects. Once these have been identified, evaluate each of
them by doing the following:
Step 1. Measure the laser beam irradiance at the closest
point of audience access. To do this, project a non-moving beam into
the venue. (Ideally, this should be done back at the studio, with
good prior knowledge of the show site. Do this at the venue only
while the room is not occupied by non-laser people or audience
members.) This beam must be the same color and power level as the
effect being evaluated. Carefully place the detector head into the
beam at the closest point of audience access. (Be aware that light
can be reflected off of the detector head, particularly with silicon
detectors. Make sure that this reflected light does not pose a
hazard to others in the room.) Make sure that the beam overfills (or
at least fills) the one centimeter detector area. If the beam
diameter is less than one centimeter, this is already an unsafe
exposure unless you are using laser powers below 15mW. Record the
value reported by the meter as "watts per square centimeter". For
example, if the meter reads 7.5mW, you would record it as 7.5mW/cm2.
Now 7.5mW may seem extremely low [see
note 1]. Who would do a show with a 7.5mW beam? Why even measure a
7.5mW beam? The answer is that in step 1, the beam is not 7.5mW, its
irradiance is 7.5mW per square centimeter. Hopefully the beam
diameter would be greater than one centimeter, and the 7.5mW would
be collected in the brightest portion of the larger beam. Tens of
watts of actual beam power can be used provided that the beam
diameter at the closest point of access is large enough to lower the
irradiance to an acceptable level.
Step 2. Measure the pulse-width of the effect as it
crosses the eye. To do this, project the effect into the venue and
carefully place the fast photodiode into the effect at the brightest
place in the effect. (Again, be aware of stray reflections.) The
brightest place will probably have multiple points in the image to
hold the beam in place for accentuation (e.g. at a corner or anchor
point). Using the oscilloscope, measure and record the pulse-width,
adjusting the horizontal time base as needed. (Although there are
many ways to define pulse-width, safety experts agree that the
"Full-Width, Half-Maximum" points should be used. For example, if
the pulse is 2 volts in amplitude, measure the width at the 1 volt
point.) Depending on the effect, this will probably range from
around 20 to 500 microseconds. (Make sure that the detector is not
saturated during this measurement. If the pulse has a flat top, it
could be saturated and you should use a neutral density optical
filter to reduce the amount of light striking the detector.)
Step 3. Measure the pulse repetition rate. To do this,
simply increase the horizontal sweep time until you see two or more
consecutive pulses, and measure the time between pulses. Using the
scientific calculator, compute the repetition rate by taking the
inverse of this time. For example, if you measure 16 milliseconds
(0.016 seconds) between pulses, the pulse repetition rate would be 1
/ 0.016 or 60Hz.
Now that we have collected information about the effect, we will
see how this effect stacks up against the Maximum Permissible
Exposure [MPE] prescribed by safety guidelines and government
regulations.
Step 4. Compute the single-pulse Maximum Permissible
Exposure [MPE] for this effect. This is the safety guideline or
government regulation prescribing the maximum amount of irradiance
(laser power density) that is considered safe for a given
pulse-width. To compute the single-pulse MPE [see note 1] (in Watts
per square centimeter), raise the pulse-width (in seconds) to the
3/4 power, multiply the result by 0.0018 and divide the entire
result by the pulsewidth (in seconds). For example, if the
pulse-width is 100 microseconds (0.000100 seconds) the calculation
would be (0.000100) Ύ X .0018 / 0.000100 = 0.018 W/cm2
or 18 milliwatts per square centimeter. (To do this with the
scientific calculator provided with Microsoft Windows, enter 0.0001,
press the X^Y key, enter 0.75 (equivalent of 3/4), press *, enter
0.0018, press /, enter 0.0001, and press =.) If the irradiance
measured in Step 1 is greater than the single-pulse MPE, stop right
there -- the effect is not even safe for one pulse of laser light
(one scan across your eye) and must not be performed before an
audience.
Step 5. Compute the multiple-pulse MPE for this effect.
This is a reduced version of the single-pulse MPE, based on the
number of pulses that the audience member will be exposed to. Basically, the more
pulses of light that are received by the eye, the less light allowed per pulse. To
compute the multiple-pulse MPE, multiply the exposure time (in
seconds) by the pulse repetition rate, and raise this number to the
-1/4 (negative one quarter) power. For example, if the exposure time
is 1/4 second [see note 2] and the pulse repetition rate is 60Hz,
the calculation would be (0.25 X 60) -1/4 = 0.508. (To do
this with the scientific calculator provided with Microsoft Windows,
press the ( key, enter 0.25, press *, enter 60, press the ) key. This
gives you the total number of pulses experienced during the exposure
time. Then press the X^Y key, enter
0.25 and press the +/- key (equivalent of -1/4), and then press =.)
You then multiply this factor by the single-pulse MPE to derive the
multiple pulse MPE. In this example, it would be 0.018 X 0.508 =
0.0091 W/cm2 or 9.1 milliwatts per square centimeter. If
the irradiance measured in Step 1 is greater than the multiple-pulse
MPE, stop right there -- the effect is not safe for the exposure
time and would have to be reduced and re-measured before performing
before an audience.
Step 6. Compute the average power delivered by this effect
and compare it to the average MPE for the exposure time. To do
this, multiply the irradiance measured in Step 1 by the pulse-width,
multiplied by the pulse repetition rate. For example, if the
irradiance is 7.5mW/cm2 and the pulse-width is 100
microseconds and the repetition rate is 60Hz, the calculation would
be 0.0075 X 0.000100 X 60 = 0.000045 W/cm2 or 0.045
milliwatts per square centimeter average power. Using the
calculation for single pulse MPE, we can find the average MPE for a
1/4 second exposure (since the exposure time is 1/4 second in this
case) as 0.25 3/4 X .0018 / 0.25 = .00255 W/cm2
or 2.5 milliwatts per square centimeter. If the average power delivered
by this effect is greater than the average MPE, this effect is not
safe for that exposure time.
It is handy to have someone check your calculations. Mistakes can
have an immediate effect on the audience, unlike calculating X-ray
exposures where your mistakes manifest themselves 20 years later.
In order for the effect to be considered safe, it must not exceed
any of the three MPE limits. In audience scanning shows, the
multiple-pulse MPE will be the most restrictive and the average MPE
will be the least restrictive. This particular example illustrated
this as the single-pulse and average MPE were not exceeded but the
multiple-pulse MPE was.
Score the Whole Show
The manual processes described above should
be repeated for as many effects as possible. Or, if time is limited,
you should measure the effects that pose the greatest hazard. These
are ones which project only a few beams into the audience or project
patterns which are small in size or look particularly bright. If an
effect exceeds the MPE, you can reduce the laser power or brightness
of the effect, or change the effect to decrease the pulse-width or
number of pulses.
You should also consider the "total
MPE" of the entire show. If all of the effects in your show were
barely below the MPE, the show as a whole would probably be above
the MPE. Since you are calculating the MPE for only specific effects
in the show, you must manually "score" the whole show.
Unfortunately, at this time, nobody has developed a
statistical method of arriving at this "total MPE". Until that time,
err on the side of safety by reprogramming effects that are "on the
edge".
Increasing divergence to reduce the irradiance
While reading this or performing measurements on your own show,
you may realize that relatively low beam powers must be used if the
beam diameter at the audience is small. This is because if the beam
diameter is small, the irradiance is high. You can decrease the
irradiance by increasing the beam diameter at the audience, which
will allow substantially higher beam powers. To do this, you will
have to use a lens or collimator. Multiple watts of laser power may
be used if the irradiance is kept to a reasonable level by expanding
the beam.
A simplified approach
After performing manual analysis over and over, on hundreds of
effects and shows, it will be seen that in order for an Audience
Scanning show to be safe, several factors need to be in place:
- The actual scanning and beam modulation need to happen at a
rate fast enough to keep the pulse-width experienced by the eye
around 1millisecond or faster.
- The maximum irradiance of a beam measured at the closest point
of audience access needs to be somewhere between 5mW/cm2
and 10mW/cm2.
If you accept these two factors, then a simplified approach can
be used to evaluate audience scanning safety. The simplified
approach involves measuring the irradiance of a non-moving,
non-modulated beam at the closest point of audience access, in a
manor similar to Step 1 above. The beam must represent the highest
power level that will ever be found in the audience, thus allowing
you to gauge the maximum irradiance that will ever be experienced by
the audience. For an RGB laser projector, this should be a white
beam.
Note that with modern software, it is
often difficult to get a non-modulated beam, since most of the time,
modern software will always be modulating the beam for some reason
-- for example, during inter-frame blanking periods, whether an
animation is being projected or not. Therefore you must consult your
software company to find out how to get a non-moving, non-modulated,
and essentially full-power beam out of the software so that this
measurement can be performed.
Once the irradiance of a non-moving,
non-modulated beam is measured, it must be between 5mW/cm2
and 10mW/cm2. If the beam power is higher, you will need
to reduce the power coming out of the projector, or increase the
divergence to achieve an irradiance level between 5mW/cm2
and 10mW/cm2. And of course, this simplified approach can
only be used AS LONG AS a reliable system is in place to ensure that
the two factors mentioned above are not violated under any
circumstances.
(The rigorous mathematical basis for this simplified approach is
not presented in this article, however, it should be noted that this
is also the consensus of the Thesis on
Audience Scanning Risk Assessment by John O'Hagan. The Theses
can be consulted for more detailed information.)
Keeping the Show Safe
Just because the show passes all of the evaluations now, does not
mean that it will stay that way. A number of things can happen to
make the show unsafe. Examples of these include: sudden increases in
beam power, and something about the projection system failing,
stopping the
scanning from occurring including computer, cabling or scanning
system failure. You must consider reasonable failure modes and
provide control measures (such as scan fail safeguards) to limit the
consequences. Pangolin's PASS system
was designed to monitor the projected beam power as well as the
scanning system and other projector-related systems and ensure that
these are operating within a safe level.
A Reward for Your Hard Work
The next time you have an opportunity to view audience scanning
shows, pay particularly close attention to your vision as various
effects cross your eyes. Effects that appear to leave a strong
afterimage are not pleasant to experience and detract from the whole
show. When this happens, your eyes will be too busy recovering from
the last effect to enjoy the next one. However, effects that leave
little or no afterimage are very beautiful and fun to experience. In
this case, your eyes will say "Wow! I made it! And I can continue to
enjoy the show!"
It turns out that effects that exceed
the MPE will generally cause afterimages, while effects which do not
exceed the MPE will not. As artists, you can learn from the MPE
measurements and create shows that are safe and enjoyable by all.
Endnotes
Note 1. Throughout this article, the MPE values expressed
are from the ANSI Z136.1 Standard for Safe Use of Lasers. Although
this is technically different from other international safety
standards, the main difference for visible wavelengths is the units
of measure. For example, while the ANSI standard uses Watts per
square centimeter, some other standards use Watts per square Meter.
However, these standards are surprisingly in agreement as to the
actual "Exposure Limits". You should refer to the laser safety
standard for your country and seek regulatory advice. In some
countries such as Sweden, it is illegal to scan the audience with a
laser beam.
Note 2.When determining the exposure time for an effect,
you should take into account how long the effect will linger in
place. For example, fan effects and tunnel effects that are moving
will sweep past the eye very quickly, and thus, the exposure time
would be very short -- perhaps on the order of just a single sweep.
But fan effects and tunnel effects that are not moving will scan
past the eye multiple times, causing a longer exposure time. When in
doubt, a quarter second (0.25 seconds) may be used since humans will
"avert" the beam (by blinking or turning their heads), and one
quarter second is the universally-accepted natural aversion response
within laser safety standards.
Copyright 1997-2008, Pangolin Laser Systems. All rights reserved.
Reproduction in whole or in part in any form or medium without
express written permission from Pangolin Laser Systems is
prohibited.
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