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How much laser power do I need?

Introduction

Depending on the type of laser effects desired, different output power is needed from the laser(s) used at a show. Your effects could look spectacular or wimpy based on what type and power of lasers you are using.  This article gives some guidelines based on practical experience for the power levels need to make different types of laser effects in different venues.

Classes of effects

The two main classes of effects used in laser shows can be divided into screen effects and beam effects.  Screen effects are any effect that is projected onto a surface for viewing by the audience,  Screen effects include graphics, animations, abstracts, cycloids, optical and grating effects.  Beam effects are those effects created by projecting the beam through the air and include static and dynamic beam effects, sheets, cones, tunnels, fans, etc., and grating effects.
Generally speaking, screen effects require much less power than beam effects as the projection surface serves to reflect the light towards the viewers eyes.  Beam effects take considerably more power as they have to illuminate many particles of dust or haze suspended in the air in order to make the beam(s) visible to the audience.

Screen Effects

As motioned above, screen effects take considerably less power to produce than laser beam effects.  To make a good screen image, you need a contrast ratio of about 100:1.  This means that the image has to be 100 times brighter than the background [projection surface].  To make a high quality laser image, one that gives the illusion of being almost three dimensional and floating off the screen, you need a contrast ratio of 250:1 to 300:1.
Fortunately, laser graphics inherently conserver photon energy when compared to other forms of display such as slides, movies or video.  In those forms of display, the entire image generating surface [the film or light control element in a video projector] must be fully illuminated with an even light, even though part, or even much, of the mage on the screen may be black.  Vector laser graphics only put light where it is needed to form the image.
In a controlled lighting situation such as a theatrical setting where almost total darkness can be obtained, large laser images can be made with relatively low power lasers.  Graphics up to 20 X 20 feet [6 x 6 meters] can be made with as little as 250 mW of laser power.  Note that the 250 mW is light delivered to the screen; you will have to start with a higher power laser to take into account losses in the PCAOM and optics. Naturally a smaller image can be made with less power.
As the light level in the venue increases, higher laser powers are needed.  In an industrial trade show situation where an image must be projected in a venue lit with fluorescent or mercury vapour lights, 3 to 5 watts may be needed to make an image 10 X 10 feet [3 X 3 meters].
The quality of the projected image is also dependant on the projection surface.  The ideal projection surface for laser is not a white screen [unless you have a very low power laser and need to reflect as much light as possible].  White screens are highly visible to the audience as they reflect any scattered light in the venue [even exit signs and isle lighting in theatrical settings].  White screens also tend to make the image look slightly 'washed out' and it is more difficult to get the 'image floating in front of the screen' effect.
The best projection surface is one that is about 18% gray so that some light is absorbed and little of the ambient light in the venue is reflected.  This means that the typical rear projection screen made of translucent gray plastic is the preferred surface - even in front projection applications.
Often one has to work with existing surfaces such as the side of building which are less than optimal.  In those cases, one has to scale the laser power upwards to make an image with a high contrast ratio, and may have to adjust the colour palette of the graphics system so as to compensate for the background colour of the building surface if it is not white to gray in colour as otherwise the colours in the image will be affected by the projection surface.

Beam Effects

It is possible to make good beam effects with as little as 250 mW of laser power provided you have absolute darkness and a good haze.  In conditions where there is other lighting in the room, such as in clubs and bars, at least 1 watt of power and a good haze is required to make acceptable beam effects; 2 to 3 watts are needed to make impressive beam effects.  For truly spectacular beam effects in those conditions, 5 or 8 watts of laser power would be required.
For beam effects in well lit trade shows, large arenas and stadiums, more laser power is required.  In an indoor venue, 4 to 6 watts will make good beam effects with a proper haze provided ambient lighting is not high.  For outdoor venues where the haze can not be counted on to say in place, 10 to 20 watts or even more would be required.
The visibility of beam effects is also dependant on two other major factors - viewing angle and haze.  The beam should be as close to the viewing axis of the audience as possible while respecting safety requirements.  This means that the audience should be facing the source of beam projections and the beams should be as close over their heads as is legally possible given the power level of the laser.  Note that static beams should never be projected into an audience and scanned beam effects should be of low enough power density so as not to cause afterimages in the viewers eyes. [See the article entitled Making Shows Safe And Enjoyable for more information.] 
The other factor affecting beam visibility is the quality of the haze in the venue.  Water based machines make impressive thick clouds of smoke but these both block the beams, and dissipate rapidly. A fine, almost invisible, haze is the best for beam effects as it does not block the beam diminishing it's visual effects, but rather makes the beam visible along more of it's length.  A professional quality hazer such as the MDG MAX3000 or similar with a fan would be adequate for venues up to a small of medium sized arena.  For larger spaces, the MDG MAX5000 with a fan, or even a few MAX5000s with fans would be required.  The best strategy is to turn on the hazer(s) on before the audience arrives and to keep them running at a slow but steady rate during the presentation, using the fans to disperse the haze.
It is preferable to blow the haze to the side of the laser beam paths and allow it to disperse throughout the venue unless 'texture' effects in the beam are desired.  In that situation, it is best to place a hazer below and in front of the laser position and blow the smoke out below the beam path allowing it to rise into the beams by convection.  [For more information on hazers see Smoke Machines Vs. Hazers.]

NOTE:  The power levels mentioned above are presuming the use of an argon or white light laser system.  Other types of lasers such as Copper vapour and 532 YAG lasers have much higher apparent brightness due to the sensitivity of the human eye so will appear brighter for the same power levels mentioned.  YAG lasers of three times less power, or CV lasers of half the power mentioned above can be used to make these effects..

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