Laser and Optics
The
laser is a >5W green laser similar to those I've used in the past.
It is housed in a box near the installation and a beam is piped to the
installation via an underground tube. At the center of the structure a
splitter and mirror array breaks the beam into four, which are sent up
to mirror boxes, then horizontally to the next mirror box over, and
finally to beam absorbers at the top. While the octohedron could have
been made entirely with one beam (which I have done in the lab), such a
scheme has problems with beam spread, loss per mirror bounce and
stability.
The unusual thing about this laser installation, and what makes it more participatory, is that people get to be near the beams. To prevent people from actually playing with the beams, there is a special detection and blocking mechanism. A low power red laser beam larger than the green beam in diameter is propagated along with the green. This red beam is detected at the beam absorber ("beam dump") at the end of the beam paths. If the red beam is interrupted or even reduced partially in power (over a period of less than a few seconds), a fast shutter closes off the green beam. It is restored after several seconds, so that people can't get interested in flashing the beam. An operator will also be present to keep people in line in a friendly way. This sort of arrangement worked for The Grid, an installation in 2001 that was approved by the regulatory agency for laser shows. There, only the green beam was detected.
At each vertex of the octohedron, crossing beams define the end points. This is made possible by reflecting each beam twice in a mirror box. These boxes have four windows and contain four manually adjustable mirrors. The windows are partially protected from direct dust blasts by covers open on one side. These covers also prevent reflections off the windows from shining out in random directions. A thin layer of dust will form on the clean windows, and then no more, as I have found in previous years.
The mirror boxes are mounted to the support structure with long bolts or threaded rods which can be adjusted to orient the box correctly even if the structure is not exact. The rear of the box is a 1/4 or 3/8 aluminum plate, while the rest can be thin aluminum. There are convenient commercial box construction materials like 80/20 that could be used to make these octagonal boxes with standard parts plus special cut sheets.
The unusual thing about this laser installation, and what makes it more participatory, is that people get to be near the beams. To prevent people from actually playing with the beams, there is a special detection and blocking mechanism. A low power red laser beam larger than the green beam in diameter is propagated along with the green. This red beam is detected at the beam absorber ("beam dump") at the end of the beam paths. If the red beam is interrupted or even reduced partially in power (over a period of less than a few seconds), a fast shutter closes off the green beam. It is restored after several seconds, so that people can't get interested in flashing the beam. An operator will also be present to keep people in line in a friendly way. This sort of arrangement worked for The Grid, an installation in 2001 that was approved by the regulatory agency for laser shows. There, only the green beam was detected.
At each vertex of the octohedron, crossing beams define the end points. This is made possible by reflecting each beam twice in a mirror box. These boxes have four windows and contain four manually adjustable mirrors. The windows are partially protected from direct dust blasts by covers open on one side. These covers also prevent reflections off the windows from shining out in random directions. A thin layer of dust will form on the clean windows, and then no more, as I have found in previous years.
The mirror boxes are mounted to the support structure with long bolts or threaded rods which can be adjusted to orient the box correctly even if the structure is not exact. The rear of the box is a 1/4 or 3/8 aluminum plate, while the rest can be thin aluminum. There are convenient commercial box construction materials like 80/20 that could be used to make these octagonal boxes with standard parts plus special cut sheets.