Detailed Laser Safety - Safety Archives
An Electronic Approach to
To improve the safety of laser shows, especially in jurisdictions where audience scanning is permitted, it is desirable to monitor the behavior of the scanners electronically. If a malfunction is detected, then the system can be shut down. This paper covers the theoretical considerations of such systems.
Laserists depend on their scanners to create graphics, animations and scanned beam effects. If the scanning system were to malfunction or fail, a high power beam could potentially be projected into an unsafe area and possibly injure spectators. The scanners are electronic devices and are thus amenable to hardware monitoring circuitry that can take action to shut down emissions far faster than the human operator. Such electronic safety systems are very useful even in situations where no audience scanning will take place.
Professional scanners are equipped with position
detectors as these are necessary for the precise operation
required to produce images. The position of the
scanner is detected and compared to the input signal.
An error is then generated and this is used to correct the
position of the scanners.
Diagram 1 shows a block diagram of how such hardware
might work. The output of the detector is used to
control a small relay that is the last item in line before
the shutter. When the detector senses that the scanner
has failed, the relay opens and the spring or gravity loaded
shutter closes preventing projection of the beam. The
output of the detector should be high when the scanner is
functioning correctly so that it holds the relay in.
In the event of a power or other failure, the relay will
open closing the shutter to provide an additional measure of
Diagram 2 shows and enhanced system that can monitor both
the X and Y scanners. This is accomplished by adding a
second set of signal conditioning and detection circuitry
along with simple logic. Given that the output of the detectors
are high when the scanners are functioning, the logic uses a
simple IF X=1 AND Y=1 THEN Output=1 rule to keep the relay
pulled in. If either scanner fails, the relay opens
and the shutter closes.
Diagram 3 shows how this problem might be addressed. By adding a small delay to the relay control signal, effects where the scanners stop moving, but yet where the projected effect is still safe, can be accounted for. The delay circuit might be something very simple such as a capacitor or something more complex where more accurate control of the delay timing is possible. The time constant of the delay would have to be selected such that safe effects could still be projected, but scanner failures would be detected and the shutter closed.
The systems described above are limited in their
application due to a number of factors. They do not
take into account the length of time it takes for the
shutter to close. This could vary greatly from system
to system due to the type and speed of the device, and the
height above the beam. Even in the best case where an
actuator driven shutter is positioned just above the beam,
the delay between scanner failure and the extinction of the
laser output could be 1/10 of a second or possibly
more. In an audience scanned effect from a medium
power laser where the scanner fails while the beam is in the
audience zone, this could be enough time to cause vision
One such device on the market is the CatSafe and
CatSafePro from MediaLas in Germany. These systems
incorporate microprocessor controlled logic to detect
scanner failure. In addition, the CatSafe systems
offer a adjustable Safe Area Window (SAW). This allows
the user to set areas in which the scanners can remain
motionless for targeting bounce mirrors or outboard effects,
without causing the PCAOM to blank the beam.
To improve the safety of laser shows, especially in jurisdictions
where audience scanning is permitted, the behavior of the
scanners should be monitored electronically. The monitoring
circuit would extinguish the laser beam if the scanners
fail. Basic systems which detect scanner failure and
close the shutter are useful in adding an extra measure of
safety to the show but may not react fast enough to prevent
vision damage. Professional systems use sophisticated
microprocessor control of the PCAOM and take into account
conditions where the scanners have stopped moving and yet
the effect is still safe.
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