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"Lovelight" Laser Projector

The theater laser projector was built around a Spectra-Physics model 164 ion laser. Special output mirrors were used that allowed lasing to occur simultaneously at red, yellow, green, and blue wavelengths:Model 164 Ion Laser

Blue           476 nm
Blue          483 nm
Green        521 nm
Yellow      568 nm
Red           647 nm

The raw beam, which appeared white to the eye, was dispersed with a high refractive-index prism that achieved a separation of about 2 degrees between blue and red. The two blue beams were so close that both were used; also, one of the blue beams was much brighter than the other. Both the laser head and power supply required a continuous flow of cooling water at more than 2 gallons per minute.Model 164 Plasma Tube Because the electrodes in the laser plasma tube (shown at left) were in electrical contact with the cooling water, they were prone to early failure from corrosion. After the first laser tube failed, a special heat exchanger system was built to isolate the laser from city tap water. It used a multi-stage centrifugal pump to circulate deionized water at about 35 psi  pressure. The laser required 3-phase electrical power at 220 volts and 35 amperes per line. The total light output was about 0.7 watt. Thus, the energy conversion efficiency of the laser was approximately 0.01%.
 

Projection System
 
The projection system was housed in an enclosed relay rack cabinet about 2 1/2 feet square by 6 feet tall. The cabinet was bisected internally by a 1/2 inch thick aluminum plate. The nearly 4 foot long laser head was mounted vertically on one side of the plate, pointing downward. The laser beam was reflected through a hole to the opposite side of the plate, on which was mounted the projector's optical components. As shown on the diagram above, the white laser beam was refracted into its component colors by a prism. The four individual colored beams were then reflected through four acousto-optic modulators. The modulators, made by Isomet, allowed the intensity of the individual beams to be varied electronically at frequencies from DC to more than 5 kHz. After the beams passed through the modulators, they were directed through various special effect devices to four pairs of horizontal and vertical scanners.
 

Playback System

 
The projector rack cabinet also housed the Hewlett Packard instrumentation recorder, galvanometer drivers, and other electronics. As shown in the diagram above, the show was played back from the instrumentation recorder, demodulated, and sent to the individual scanners and acousto-optic modulators. The special effects control signals were demodulated in similar fashion, and then converted to logic-level signals. The audio deck was not housed in the rack cabinet, but was synchronized electronically by means of a signal recorded on the FM show tape.

Next: Special Effects