"Virtual advertisements" are beginning to pop up on TV screens during sporting events, bringing in new revenue for broadcasters and more exposure for advertisers.

Unlike stadium billboards ads or television "burn-ins," which show scores and stats to viewers, virtual ads appear to be at the stadium but can only be seen by viewers at home. The ads get bigger or smaller as cameras zoom in or out, but are blocked - just like real billboards - when the action moves in front of them.For years, moviemakers have achieved these special effects through the magic of post-production work. Now, using the same technology that allows missiles to lock on to targets using laser imaging, the effects are being achieved on live TV.

"Whatever you can imagine, we can do," said Lawrence L. Epstein, chief financial officer of Lawrenceville-based Princeton Video Image, which provides virtual ads for the San Francisco Giants, San Diego Padres and Phila-del-phia Phillies baseball broad-casts.

PVI, which has used the technology on about 700 live broadcasts since 1995, will expand its use this month on several National Football League exhibition games.

Imadgine Video Systems, which also offers the technology, is preparing to superimpose ads on the mat at World Wrestling Federation matches. Ideally, he said, the ads look so real "the viewer shouldn't even know it's virtual."

One advantage of the ads, PVI officials say, is that different billboards can appear in different markets on the same physical space. The Giants and Padres already have such an arrangement.

Advertisers like the technology because viewers don't channel surf while a batter is at the plate or during an extra-point attempt. And for broadcasters, it's a new revenue stream.

The Phillies charge $125,000 a season for a half-inning spot behind home plate during their 75 televised home games. The price goes up to $20,000 a half-inning for a nationally televised game. Real billboard ads cost $85,000 to $225,000 a year.

While the companies that provide the technology say its future is boundless, some broadcasters are proceeding with caution. Bob Jeremiah, ESPN's vice president of special sales, said the network doesn't want the ads to jeopardize the appearance of the programming.

"We are satisfied that in certain situations the technology can be used safely," he said.

ESPN, which used the technology during the X-Games and a Yankees-Mets baseball game, plans to use it again for the Braves-Giants matchup Sunday. The network is also considering it for basketball, ice hockey, boxing and auto racing.