Optical Engineering for Beam-Matrix Art
With large-scale outdoor laser installations, artists must pay attention to beam quality, stability, path length, atmospheric conditions and safety, as well as artistic requirements.
Artistic use of
lasers can take various forms. The best-known form is the laser show, which is typically
accompanied by music, and uses scanning, diffraction and other effects to project
constantly changing patterns on screens or smoke. Another type, a beam matrix, describes
geometric figures using the glowing beams as line segments. Nam June Paik is an
artist who uses this medium, although he may be better known for his work with video.
Beam matrix is also the medium I’ve adopted
for my art installations at Burning Man, a yearly arts festival in the Nevada desert,
where I’ve designed and installed four large-scale laser systems.
The festival assembles a temporary
city of 30,000 open-minded participants and is a perfect venue for experimental
high-tech art. The location is a harsh natural environment — a huge expanse
of flat, dusty lake bed desert called Black Rock —with a totally dark night
sky, despite dim light from the makeshift city.
My first installation was The Tetrahedron,
a 20-foot-tall, three-dimensional figure made from six beams supported on four towers.
Beaming Man was the next, a 4000-foot-long human figure comprising eight line segments
suspended 30 feet off the ground by towers, and powered by three lasers. Then came
The Grid, a crisscrossing matrix of beams one foot off the ground, covering a 40-by-50-foot
rectangular area with a grid of squares (Figure 1).
Figure 1. The Grid laser installation created a square matrix near the desert floor.
Last year, my Laser Beacon installation
emitted four 4000-foot-long beams to the north, south, east and west from a 40-foot-tall
building in the center of the city. The project used one laser split four ways (Figure
2). In all cases, the beams were stationary.
The visual impression created by laser
beams is unique. The beam is not an object in the usual sense, because what is seen
(air molecules, dust or water vapor) is present only momentarily, while the zone
of illumination is perfectly steady. This gives the illusion of an ethereal, perfect
“object” that has little connection to the real world. Drawing straight
linear figures across thousands of feet with no support seems an otherworldly feat.
The artist’s job is to employ providing a strongly evocative visual and emotional experience. While the artistic
aspects of these installations may evade rational explanation, the engineering principles
are fairly straightforward.
To achieve laser artworks beyond a
few simple geometric figures requires using many mirrors to reflect the beam in
a complex geometric pattern. Because each mirror reduces power, beam quality and
pointing stability, the total number of mirrors in each beam path should be minimized.
If each mirror in a beam path has reflectivity R, after n mirrors,
the power drops by Rn.
Figure 2. A beam from the Laser Beacon shines from a
lighthouse and passes through a temple to end on a target
A decrease of about 50 percent is acceptable
because the logarithmic sensitivity of the human eye will slightly dim the last
segment of a beam path as compared with the first. The direction from which the
viewer sees the beam will make more of a difference in apparent brightness, so small
variations in power don’t make much of an impact on the viewer’s impression
of the overall piece.
The random errors in stability and
flatness will accumulate as the beam hits each successive mirror, increasing beam
divergence. The system will then require either larger (and costlier) mirrors or
shorter beam paths to avoid clipping and the reduction of apparent brightness caused
by increased diameter. Mirrors with quarter-wave or better flatness help maintain
Designers use highly stable mirror
mounts and other robust hardware to help minimize beam-pointing instability. Because
of the specialized nature of these installations, most auxiliary hardware must be
custom-designed and fabricated.
The Beaming Man installation required
good pointing stability for mirrors mounted on 30-foot antenna towers. A laser at
the base of each of three towers was split into two beams that were directed up
the tower to two remotely controlled mirrors. The beams had to travel 2000 feet
to a 4-foot-square target on another tower. A guy wire system designed to prevent
twisting made the towers stable enough to keep the beam on the target for many hours
with only minor daily alignment, even in strong wind.
With a large number of mirrors, it
is desirable to break up the figure into multiple beam paths. One laser beam can
be split into several, but the power per beam drops. This trade-off argues for use
of the highest-power laser available because the alternative — using multiple
lasers — would be more costly. For all of my installations at Burning Man,
I employed beamsplitters to reduce the number of mirrors per path or to reduce total
path length (Figure 3).
Figure 3. An example of a compromise between beam power and path length minimization is The Grid (shown in Figure 1), whose layout is shown here. The figure could have been described by one beam.
To maximize apparent brightness, the
beam should be as small as possible, but a small beam diverges quickly because of
diffraction, as does a highly multimode beam. Therefore, an installation requiring
long beam paths should use a high-quality Gaussian beam expanded to an optimal size
via a telescope. If the beam path length is twice the Rayleigh range, variation
in diameter will be less than the square root of two, resulting in an apparently
constant brightness. Again, the variation in the angle from which the beam is viewed
will cause larger variations in apparent brightness than this diameter change. In
practice, air turbulence causes a lot of wavefront distortion and beam spreading,
and clipping in the optics introduces diffraction, but the visual impression still
Atmospheric scattering makes the beams
visible. Rayleigh scattering (because of infinitesimally small particles like air
molecules) happens in clean air but is relatively weak. Mie scattering due to larger
particles (dust, in this case) is much stronger, but also more directional. Most
of the light scatters forward, with less scattered to the side or toward the source.
Viewed from near the source, the beam
appears brighter because the viewer is looking through a longer path in the scattering
volume. Thus, the beam is most visible going toward the viewer, somewhat visible
going away and least visible from the side. Although the four beams of the Laser
Beacon each produced less than 1 W, they appeared dramatic in photos taken with
a beam heading toward the camera, even compared with higher-power omnidirectional
The dust in the atmosphere over the Black Rock
Desert results in efficient scattering, but it can completely obscure the beam in
a dust storm. After a rain, the reduced dust makes bright beams become barely visible.
Fortunately, the sky above the desert is very dark, so relatively low-power beams
are adequate. A 1-W beam propagating 4000 feet, with most of the light hitting a
distant target, is strongly visible overhead against the night sky.
The harsh environment of the desert
includes large diurnal temperature swings (around 50 °F), rain and 70-mph dust-filled
winds. For this reason, in my installations, I used mirrors and remotely controlled
mounts at the top of exposed towers, protected by aluminum housings with motorized
doors that opened only at night. The doors were designed to minimize the dust that
wind could force into them while they were closed. They were made of aluminum because
the corrosive alkali dust of Black Rock can rust steel in minutes when wet. Also,
sealed wooden boxes with inlet filters and exhaust fans protected laser power supplies
and provided clean air cooling. The laser head and other optics were housed in a
sealed box with antireflection-coated windows. The thin layer of dust that sticks
to exposed optics appeared to have little effect beyond minor scattering.
Because the only available electrical
power at Burning Man comes from generators, and there is no water except for that
carried to the desert in containers, the lasers must be efficient and require little
cooling. Diode-pumped solid-state lasers are the obvious choice. I’ve used
both the Coherent Verdi and the Spectra-Physics Millennia 5- and 10-W models because
of their beam quality and efficiency. The lasers typically run at 5 W or less for
six hours a night during the week of the event.
And, because the lasers used in these
installations are rated a Class 4 optical hazard, all the installations were designed
with safety paramount, and variances from the Center for Devices and Radiological
Health were granted to the last three. Precautions included interlocks, optical
design to prevent stray or satellite beams, trained operators and emergency crash
buttons. Beams were always terminated on targets or beam dumps. In the case of The
Grid, beams were a foot off the ground, accessible to participants, although
with the beam expanded to about 1 inch, power density was too low to present a skin
Operators were present at all times,
preventing people from placing their heads, mirrored boots or any shiny object into
the beam. They made announcements over the public address system that educated participants
about laser safety. So participants treated the surrounding beams with respect.
One must make the trade-offs between
artistic expression and technical limitations early in the design phase of laser
art projects. Once they are built, though, the installations convey subtle imagery
to thousands of desert art fans, proving the communicative appeal of this unusual
Meet the author
Russell Wilcox of El Cerrito, Calif., is a laser
engineer with Lawrence Berkeley National Laboratory.?He is a seven-year veteran
of the Burning Man event.
- beam matrix
- 1. A geometrical arrangement of two or more light beams for use in laser shows, object detection or other applications requiring arrayed multiple beams. 2. A mathematical 2 X 2 or 3 X 3 matrix for calculating the propagation of a Gaussian laser beam through optical components.
- As a wavefront of light passes by an opaque edge or through an opening, secondary weaker wavefronts are generated, apparently originating at that edge. These secondary wavefronts will interfere with the primary wavefront as well as with each other to form various diffraction patterns.
- The successive analysis or synthesizing of the light values or other similar characteristics of the components of a picture area, following a given method.
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