The first thing you notice about Photon is that it smells odd.

It should; it's an alien planet.

The air is visible: a fog of suspended, perfumed particles crisscrossed by beams of light. Little clusters of red and green lights move around strangely: bobbing and weaving, darting in odd, abrupt changes of direction. Sometimes the prevailing darkness vanishes momentarily in a stroboscopic flash. Or a flying saucer zooms overhead, tilting and whirling and stabbing out shafts of brilliant white light like the spokes on a wagon wheel. The thick, weirdly perfumed air throbs with strange noises -- machine sounds, animal sounds . . . sounds that you cannot identify. Could they be music? It's hard to believe.

You are trapped in a video game.

It is life-size, three-dimensional and populated with real human beings carrying electronic pistols. Some are friends and some of them shoot at you. You can tell which are which by the blinking lights on their helmets.

You have paid $3.50 for 6 1/2 minutes of this experience.

For people in the Washington-Baltimore area, the planet Photon is located in the North Point Shopping Plaza, an abandoned-looking complex in Dundalk, near the Bethlehem Steel plant, where the Baltimore Beltway runs into a sort of wasteland. More than half the shops in the complex are out of business. One that is not, the Whitehouse Tavern ("Coldest Beer in Town") has a banner announcing low prices for drinks "Till Times Get Better." On the fringes of the nearly empty parking lot, dandelions go to seed in patches of uncut grass.

But inside the long, low building that houses "Photon: The Ultimate Game on the Planet Earth," all is fantasy, wish fulfilment, escape. Escape to what? To a tennis-court-size world where people dress up like Darth Vader, scurry through a futuristic maze with such landmarks as the Alien Tower, the Engine Room and the Crow's Nest and zing one another with odd-shaped guns.

There are two teams, red and green, identified by their helmets, which are decorated with a circle of small, colored, blinking lights. Reds shoot at greens and vice versa; you get points for hitting an enemy and lose points for being hit or hitting one of your own team. The points are registered automatically by a small computer strapped to your chest and attached to your pistol.

The Dundalk Photon, which opened for business last night, is the ninth in two years. The first became flesh (and specialized architecture and high-tech electronics) in April 1984 in Dallas, the home town of inventor-entrepreneur George A. Carter III. By the end of 1986, Carter expects 40 Photon franchises to be open for business, and he has already awarded franchises for 130 cities here and abroad. Each franchise gives the holder exclusive rights for a particular metropolitan area. Some franchise holders are already planning to open a second Photon unit -- for example, the Baltimore partnership, which is headed by Capt. Michael Whittaker of the Baltimore County Fire Department. Whittaker is proud that Baltimore has its Photon working before such cities as Chicago, Los Angeles and New York. "It was important," he says, ". . . because many people do not view Baltimore as a progressive city."

Carter, in town for the opening and beaming like a proud father, says that Washington is "our last major franchising area not taken up -- we just haven't connected with the right people yet." He thinks it will be "interesting" to see how Washingtonians would react to Photon if they had one. "A lot of Washington people know about it already," he says. "When the Republican convention was held in Dallas, a lot of delegates played in teams -- one state against another -- and had a lot of fun. They would book it for a couple of hours as a private party."

Photon is the first Carter idea to catch on, but not the first to catch attention. In the 1960s, while he was a student at Arizona State University, he invented a motorized surfboard, patented it and sold the production rights -- but it never got into production. "I just anticipated the market a little too soon," he says. Then there was the Snoopy (named after the well-known dog), a two-person, off-the-road recreational vehicle with special features. About 2,500 units were manufactured in the late 1960s before the manufacturer went bankrupt.

But with Photon, Carter may have hit the jackpot -- at least for a while. He is clear-eyed about the limited potential of other games and starry-eyed about the possibilities of Photon. In video games, he says, "even the less accomplished players eventually get tired of the repetitious nature of the game." But in Photon, "when you have the human brain -- or 10 or 20 of them -- spontaneously controlling the way the game unfolds, you introduce an essentially infinite variety of twists and turns." He hopes to have an international Photon tournament next year, with a $100,000 prize for the best player.

When a new Photon unit opens, he says, "typically, it will feature over 1,000 people coming in and buying memberships every week. In Dallas, after two years, we still have 800 new people per week." Membership is sold in the form of a "passport" to the planet, complete with photo, a one-time expense of $6.50 and usable at any Photon unit. With nearly 100,000 passport holders in Dallas alone, by Carter's figures, it is clear that not everyone who has a passport visits the planet regularly. Taking no more than 20 people at a time, the facility couldn't handle them.

Carter concedes there are two kinds of Photon visitors. The hard-core form teams and leagues, play every week under exotic nicknames and invent specialized vocabularies and myths. Among this group is one legendary team member who commuted from Saudi Arabia to Dallas and back overnight so he would not miss his regular Wednesday night league game. But for most people, Carter says, "we have concluded that Photon is alternative entertainment; they will come, in groups of three or four, on nights when there isn't a good movie showing."

The clientele is 80 percent male, he says, and primarily men between the ages of 20 and 25, "but there is no age limit. The oldest Photon player I know is 86." Because of the size of the equipment, players must be 4 1/2 feet tall. Hard-core players call those who just make the size limit "Ewoks."

Carter is puzzled at the low proportion of women players. "This is one sport where women have an even chance," he says. "Size and strength don't matter; what you need is fast reactions and the ability to anticipate the other player's moves."

And perhaps a special kind of madness.