BLOG: Laser Pulses; emissions and exposure
This article explains the importance of laser pulses in the context of laser show applications, and is intended to clear up some of the confusion that exists when considering exposure levels. It’s a topic I've decided to write about due to frequent number of times I meet people, many of which are seasoned laser show operators, who question why 'pulsed' MPE needs to be considered for light show applications, where most of the time, continuous wave (CW) lasers are used.
Types of Laser Output
For the benefit of the uninitiated (and those that have not had the ‘pleasures’ of working with a truly 'pulsed output' laser), lasers can be considered as falling into one of three categories based on their output behaviour when energy is applied.
- Continuous Wave Output - Denoted by a CW after the output power figure
- Repetitive Pulsed Output - Denoted by Q-CW, meaning 'quasi-CW'
- Single Pulse Output – Normally quoted in joules J
The diagram below provides an indication of how the output on each laser type behaves:
The different laser types are listed above in terms of their popularity of use in the entertainment sector, with the first type, continuous-wave (CW) lasers, being the clear leader in the laser type of choice for light shows. Applying energy to one of these devices generates a nice stable continuous output that can be used to create solid sheets of light and shapes in mid-air. Varying a modulation or control signal to a laser alters its intensity. Any modern laser show controller takes care of this behind the scenes, where slow variations are used create gentle fades, or much faster to create strobing and multiple beam impressions. It is possible modulate the CW laser at such a rate it looks as if it is 'pulsing', which technically it is, but not in the sense of a 'pulsed laser'. For this reason, even a fast strobing CW laser is still considered, in terms of device definition, a CW laser. A truly ‘pulsed laser’ does not need an external modulation signal to make it pulse.
Repetitive Pulsed lasers differ considerably to CW lasers in that their output is characterised as being a stream of very short, (typically ns), but extremely intense pulses. The pulses occur so fast that a stationary beam will look solid like a CW beam, hence where it gets the name quasi-CW (Q-CW). Most laser show operators are aware that the energy in each of the pulses of this laser type is so intense that they should never be used for audience scanning. In cases where this has been ignored, the consequences have been severe, with spectators receiving eye injuries.
The attraction with using this type laser is that they produce extremely intense beams, suitable for outdoors and large venues, which with proper care do not put the public in harms way. However technological advances and falling costs of high power CW lasers are starting to erode the value in using the higher risk Q-CW lasers, which are additionally more cumbersome to use.
Single Pulse laser types have little use in entertainment applications, for as their name suggests, output emitted by these is very brief.
So having covered the three different types of laser that exist and how their output differs, we can turn our attention to exposure and how MPE is considered.
‘MPE’, or Maximum Permissible Exposure, to give it its full name, is a figure that defines a safe limit of light that is not expected to cause harm if viewed or exposed. The limits are defined in various safety standards and regulations.
A cause for common confusion is the reference to ‘pulsed MPE’ or ‘multiple pulse MPE’, which with a knowledge of the three different output types of lasers, someone using a common ‘CW’ laser for light shows, can, (and do), mistakenly think that ‘pulsed MPE’ is only applies to ’Q-CW’ lasers. On site visits, this has lead to some blank faces when I’ve started talking about needing to consider ‘pulsed MPE’ when undertaking an exposure assessment.
However the way in which pulses from even a nice solid stationary tunnel are produced becomes more obvious when looking the following diagram that shows a beam path of a solid tunnel crossing the eye of an observer.
The diagram helps to demonstrate that when the beam momentarily crosses the eye, the time when it passes directly through the eye’s pupil illuminates the inside of the eyeball, creating a brief ‘flash’ or ‘pulse’ of light. It’s this action of eye being scanned by laser light that makes the ‘pulsed MPE’ relevant, even for scanning laser light that is continuously on.
The MPE figure, (or dose), changes depending upon the duration, (and how often), the laser light spends directly illuminating the eye. For shorter pulses of light, the limit increases. While for longer durations, the limit reduces, meaning less light can be safely viewed. For a scanning application, the pulse duration changes, usually getting shorter the further away from the laser projector the laser is viewed.
What this means in practice is that in applications where people could be exposed to scanning effects, part of the assessment process is going to involve considering the pulse duration created by the scanning beam, and how often scanned beam crosses the eye.
This article has discussed how pulses of light can be emitted from different types of laser, and how when directly viewing scanning laser beams, even from non-pulsing lasers, it creates pulsed exposure, and the need to consider pulsed MPE to check the exposure is safe.
© 2014 LVR Optical