In old science-fiction movies, the computer of the future is often a mysterious metal box with a bevy of blinking lights on the outside. In the real future, however, the lights will be inside the computer.

Optical processing -- computing with light instead of electrons -- may spur new advances in computer speed. GuilTech Research Co., in Sunnyvale, Calif., is developing an experimental version of a processor. Aerodyne Research Inc., in Billerica, Mass., may have a demonstration device by early 1986.

Optical computers may help solve an emerging problem in computer science: Researchers seem to be approaching fundamental physical limits that may impede further leaps in circuitry speed. By attaching optical processors to existing silicon circuitry, scientists can bring some speed-of-light benefits to computing. Optical processors will also be lighter and cheaper than comparable silicon processors, according to some researchers.

"We think this has the smell of something as revolutionary as integrated circuits," says John Caulfield, an Aerodyne scientist. "There's going to be so much damn money there." If the technology works -- always a dicey proposition -- the Pentagon may buy the first optical computers to put in missiles, aircraft, and the proposed "Star Wars" missile-defense system. Later, scientists and economists may use them to do huge number-crunching jobs.

In the most common design, an optical processor calculates by shining a beam of light -- whose intensity represents a number -- through a crystal. Sound waves passing through the crystal diffract some of the light, sending it off in a new direction. To multiply 0.5 by 0.75, for example, a light at half its full strength might go through a crystal that diffracts three-quarters of it.

Optical computers still rely on existing electronics for many functions. This "wall of silicon" slows down an optical computer and accounts for most of its weight, cost, and power consumption, says Caulfield. "We don't think we're ever going to replace electronics. Rather than competing with high-speed electronics, we're urging it on. The faster it can go, the faster we can go."