If you are among the thousands of homeowners and business owners who have installed a wireless video camera to monitor activities inside, beware. Someone else may be watching too.

The so-called "nanny cams," like the highly advertised X-10 Camera, are made to transmit an unscrambled analog radio signal which is sent to a base station in the home or office attached to a computer monitor or television set for viewing. However, there is no built-in security to mask the signal, which can apparently be intercepted by "hackers" up to a quarter mile away using software that costs under $250.

If you run a business and have set up a camera for security purposes, don't assume that you are the only one with access to the images. Anyone armed with a strong antenna and a receiver attached to a laptop computer, who is walking or driving past your house or office, may be able to peak inside and see what you see.

"Although there is no evidence that video snooping is widespread, it is so easy and the opportunity to do it is so great that it is a cause for concern," said Aviel D. Rubin, a security researcher at AT&T Labs, which identified the problem with the consumer product.

Unlike laws that prohibit wiretapping and eavesdropping on telephone communications, the legal system has not yet addressed the issues of tapping video signals. Therefore recourse against such video "snoopers" would be extremely difficult.

So if you own a business where security is an issue, it may make sense to look into security cameras that encrypt the images, rather than the consumer-brand cameras that do not. And if you are a homeowner, know that the camera inside your house may be sending signals outside the four walls of your personal domain.