You could tell right away that it was not one of your run-of-the-mill Washington conventions-particularly when you began reading the buttons ("Dragonriders Have More Fun"; "Take a Wookie to Lunch Today"; "Darth Vader Is a Paramount Executive").

Or when you noticed the costumes - men in long, red-lined capes and bearing the staff of a sorcerer; others in chain mail with swords 4-feet long - some with extra-large handles because you needed both hands to swing them. And one very husky fellow dressed like a medieval peasant, carrying a dangerous-looking battle-ax over one shoulder and, perched on his wrist, a no-kidding real live falcon that surveyed the scene coolly, looking slightly bored.

The falcon seemed to be the only one with that attitude. Of the 1,202 humanoids who registered for Unicon IV, the three-day gathering that ended yesterday at the Sheraton Motor Inn in Silver Spring, some showed signs of hyperactivity, and as the weekend raced along a lot showed extreme fatigue. But there was no time for boredom.

They were too busy playing games with computers, admiring the scirnce-fiction costumes of the more adventurous participants, watching films that ranged from "Masque of the Red Death" to "Farenheit 451," pointing ray guns at one another and getting autographs from their favorite science-fiction authors. They also played highly specialized trivia games ("Who were the three chief scriptwriters for "Twilight Zone'?" "Who coined the phrase 'rocket summer' and in what book did it appear?" "What was H. G. Wells' first novel?"; and engaged in deep-think panel discussions about life-prolongation, space-colonization, the evolution of new forms of government and the technical problems of writing, publishing and filming the special kind of fiction that is their opiate and stimulant.

Unicon (the name truncates and blends "university" and "convention") began four years ago as a small gathering of science-fiction fans at the University of Maryland and has been growing at what SF fans would call an exponential rate. The first Unicorn had 200 fans in attendance. Last year, after moving off the campus, it had 550 and this year that figure was more than doubled.

Unicon is only a small part of a massive phenomenon; meetings like it are happening more or less constantly around the country. One was held in Philadephia the same weekend, and there will be another in December, not to mention one in Pittsburgh in September, Richmond in October, Albany in November, Penn State at the end of July, New York City next weekend. Baltimore had one a few months ago, and the Silver Spring Sheraton will have another one focusing on "Star Trek" the first weekend in August. Science fiction is booming, and the fans are coming out of the closet in rapidly growing numbers.

Part of the reason for this sudden growth could be seen in some of the costumes which, improbable as it once would have seemed, almost any American today would recognize: Darth Vader, Princess Leia and Chewbacca the Wookiec, allfrom Star Wars - not to mention one of those little people in brown-hooded cloaks with luminous eyes who trundled around the desert peddling secondhand androids. Science fiction, in its mass-market film incarnation, has hit the American public with the irrestible impact of an idea whose time has come.

Interestingly, nobody there was dressed like anyone from "Close Encounters of the Third Kind" - the aliens in that show were too dimly, fleetingly, glimpsed to be imitated. It would be stretching a point, perhaps, to say that most of the males present were dressed like Richard Dreyfuss in that film, since his costume was totally nondescript.

The surge in science fiction's popularity was one of the recurring subjects of discussion during the three-day convention, and some of the old-timers present in the mostly colleged group viewed the development with mixed feelings.

Writer-editor Frederik Pohl, who spoke about his "Four Decades of Editing Science Fiction," noted with awe that Larry Niven and Jerry Pournelle had sold the paperback rights to their "Lucifer's Hammer" for a quarter of a million dollars and added a bit wistfully, "that's more money than I have paid to all authors combined on all the science-fiction magazines I have ever edited.

The trend in science-fiction films, he noted with a shade of distaste, is to exploit special effects. "Science fiction seems to me to appeal to the fore-brain, the analytical part of the mind not all of it, but the best parts of it," he said. "While I really enjoy the fact that all the TV and film producers of the world are discovering science fiction and laying money on it, I don't really expect that any of them will be as exciting and revelatory as some great science-fiction novels."

Novelist Theodore Sturgeon, who was the convention's guest of honor, struck a similar note. Asked what he thought would be the American people's reaction if we made contact with aliens from another planet next week, he remarked: "If we wanted to drive them away, we'd show them 'Close Encounters'."

"I've never read a good science-fiction story that I didn't learn something from," Sturgeon said. "At the heart of every good science-fiction story there is a scientific idea, a problem, a bit of knowledge. If you can take away that idea and still have a story, it isn't science fiction. You can put a cowboy story on Mars rather than in the American West, and it will still be a cowboy story."

A highlight of the convention was a satellite television link with the West Coast, where Hollywood animators and writers Larry Niven and Fritz Leiber dialogued with the Washington audience.

There was a special irony in the uses of a satellite, because the idea for this kind of long-range communication was one of the many that originated with a science-fiction writer. Arthur C. Clarke published a detailed description of how it could be done a decade before it became a reality, but was unable to patent the idea because he could not supply a working model. Clarke's original vision was that the Chinese might put a satellite in orbit to enable them to telecast pornographic programs that would undermine the intelligence of the American population. This never materialized, but in other details Clarke's idea was remarkably accurate.

So were the ideas of other pioneers who wrote about lasers, space ships, television, submarines and genetic surgery long before they became realities. Sturgeon referred to such ideas when he said that "science fiction is the only known medicine against future shock."

A similar idea may be implied in the slogan on the most popular T-shirt at the convention: "Reality is a crutch for persons unable to handle science fiction."