Complete the form below and we’ll be in touch.
(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:
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.
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) ¾ 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.
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".
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.
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:
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.)
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.
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.
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.
Register as dealer,
for discounted laser pricing.
Complete the form below and we’ll be in touch.