Laser Deflection System
Drawing/diagram of Laser system by Elsa Garmire. Figure G1
A Technical Description of Laser System
by Elsa Garmire
LASER DEFLECTION SYSTEM
This display in the Clam Room is generated by deflecting laser light with signals from tape recorders. This programmed deflection has its source in a powerful multicolor krypton laser. This one-watt light source produces four strong colors simultaneously in a single beam .05 inch in diameter. Such a beam is so tight that it does not double in size until it has traveled fifteen feet.
The white light beam emerging from the laser is immediately sent through a prism which splits it into separate colors (see Fig.Gl). Each colored beam traverses a two-dimensional light deflection system composed of a pair of pivoting mirrors placed at right angles to each other. The mirrors used here are called optical galvanometers and are turned by electric signals. Using very small mirrors, deflection rates up to 500 cycles, and angles to 40° are achieved. The large angular deflection produces the ten-foot display at a distance of twelve feet.
The programmed light is displayed on a translucent plastic dome, 10' in diameter, placed upside down in the ceiling. The translucency allows the laser light to shine down upon the visitors, bathing them in constantly changing coherent laser light. The actual position of the laser deflection system is overhead in the ceiling and a large plane mirror is used to reflect the light down onto the plastic dome.
Light deflection at acoustic frequencies is possible by use of very small mirrors: a circle .12" in diameter followed by a square twice as big. Such mirrors can be driven directly by amplified tape-recorder outputs. Four 2-channel tape cartridge players are used to provide eight sets of stereo information. A matrix switching arrangement allows from one to four sets of stereo programs to drive the system. If only one stereo program is used, the same image will be produced by all four laser colors, but adding channel and phase reversal can invert these images up-down and left-right in several possible permutations.
Specially recorded tape cartridges were prepared for "playing" in the display, (see Appendix D). These can be made audible if the programmer desires, through the sound loops in the Clam Room floor. Furthermore, the switching matrix can accept signals from the microphones and other audio sources in the Pavilion, producing their visual display.
The laser, prism and galvanometer mirrors are in the ceiling of the Clam Room. The controls, laser power supply and its water-cooling source are in the control room and connected to the laser by a 25-meter umbilical cord. The tape recorders and galvanometer mirror amplifiers are in the control room so that the tapes which produce the display are easily changeable.
Light deflection by galvanometer mirrors is a convenient method for handling large amounts of laser power and for producing large scale displays in a short distance. A galvanometer is a scientific instrument used for measuring electrical currents. In an optical galvanometer, a permanent magnet is attached to a mirror which is free to pivot inside a current-carrying coil. When electric current is passed through the coil, electromagnetic forces cause the magnet (and attached mirror) to line up with the coil. A restoring spring returns the mirror to its original position when the coil carries no current. In the laser display, programmed
signals are used to produce the desired mirror rotation and consequent light beam deflection.
The krypton laser is essentially a discharge tube (similar to a fluorescent light bulb) filled with a rare gas called krypton. Special windows on the ends of the tube allow the light inside the tube to be reflected back and forth from special mirrors. At one end of the tube the mirror is only partially reflecting and transmits a pencil beam of intense light, normally in nine colors. By careful selection of mirrors and addition of a small amount of another gas, argon, a greater power was produced in four balanced colors: red, yellow, green and blue. The laser "head", or package which contains the laser, 15 x 20 x 120 cm long, is small enough to fit in the ceiling and is connected by a 25 meter umbilical cord to its power supply in the control room.
Laser Deflection System
by Lowell Cross
The laser deflection system represents a sophisticated step forward in a continuing project of investigating the inter-relationships among sounds and visual images. The use of a high-power ion laser as a generating source has been especially significant for the following reasons:
1) The four principal beams provide highly defined images with intense pure spectrum colors (red, yellow, green, blue) that can be independently correlated to sound events in a multiplicity of ways
2) Very large scale projections are possible, owing to the sharpness and intensity of the laser outpu
3) The coherence of the light offers numerous additional benefits, including effects from diffraction, interference, "scatter", and the observation of the moving beams through particles in the air.
At the present state of technology, the production of laser images from information in the audible range requires some form of acoustical deflection of the laser beams. The necessary presence of transducers with moving parts (in this case, the lightbeam galvanometers) proved to be the greatest challenge in the development of the system. While Coherent Radiation Laboratories' Model 52K laser was only slightly modified for our application, major design work was essential in the production of the galvanometers.
The major obstacles in installing suitable galvanometers in this system were:
1) No manufacturers previously supplied galvo pairs for light deflection in the X-Y co-ordinates, until Bell & Howell agreed to custom-build units to our specifications
2) To optimize the system, separate X and separate Y galvos had to be specified
3) The mounting configuration required extremely careful attention to design and manufacture to permit close proximity of the mirrors, yet with no obstacles in either the incoming or deflected beam paths, and finally
4) A compromise between acoustical bandwidth and the mass of sufficiently large mirrors was unavoidable.
Throughout the design and construction stage, my invaluable collaborator, Carson Jeffries, and I strove to develop a system that was as flexible and universally applicable as possible. The special X-Y signals that David Tudor, Carson Jeffries and I have recorded for driving the system are available for use as sounds for the floor loops in the Clam Room and elsewhere. The system can accept live audio information through input jacks on the control panel. Additional outputs have been incorporated to enable programmers in the control room to monitor each component in the display with an oscilloscope. Furthermore, we have left space for the future installation of laser light modulators and polarizers, which would permit a variety of Z-axis (intensity) modulation effects. There are also provisions for subsonic (0.01 to 10 Hz) generation of kinetic diffraction displays, for the generation of surfaces through mist, fog, or smoke, and for the use of the system as an interactive instrument for live performance.