Optical Engineering for Beam-Matrix Art

Russell Wilcox

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 at right.

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 beam quality.

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 seems perfect.

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 lighting.

Earthly matters

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 hazard.

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 art form.

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.