Think twice about putting cheap "nanny cams" under the tree this Christmas.

For $100 or less, the tiny home surveillance cameras let you keep your eye on your kids, your nanny, and rooms in your home, but they also allow anyone outside your home to just as easily peer in.

Heavily advertised on the Internet and by technology and electronics stores, the wireless surveillance systems are typically equipped with a video camera-transmitter you can place anywhere in the home. The cameras transmit sound and video on an unscrambled analog radio signal. A receiver plugs into your TV, DVD or other video source to capture and display the images.

Earlier this year, Dr. Aviel David Rubin and a team of research scientists from AT&T Labs-Research in Florham Park, NJ, revealed just how easy it is to tap into the transmitter's signal and display the image on a mobile laptop computer.

In a drive-along demonstration for NBC Nightly News, Rubin used a simple hand-held directional antennae connected to a laptop with a video card to see, on his laptop's monitor, what a home owner's wireless camera saw -- kids happily playing and none the wiser.

Directional antenna are commonly available in electronics stores for less than $100, but Rubin intercepted the same video signal using an antennae he constructed primarily from an empty Pringles potato chip can. The can, with chips, retails for about $2.

The home owner consented to Rubin's demonstration, but generally, it is not a crime to intercept wireless video signals. Wiretap laws generally apply to intercepting sound, not video.

"If you are picking up the video signal without the sound, the only possible coverage under federal law is if the signals are considered electronic commerce that is interstate commerce," according to Clifford S. Fishman, a law professor at the Washington, D.C.-based Catholic University of America Law School. Fishman is an expert on search and seizure, electronic surveillance and evidence.

"Under some state laws there's a common law provision that provides for 'quiet enjoyment' and as part of the law's privacy provisions it might apply (to forbidding intercepting video signals), but you'd have to look at each state law. The best you can say is no one is sure and we won't know until a case comes up," Fishman added.

Even if someone intercepts the video signal to case your home for valuables and then goes on to burglarize your home, the intercepted video images could be admissible as evidence, but the act of intercepting your video signal would not be used to bring criminal charges.

"If the interception is lawful, the use of it is not a crime," said Fishman.

Wireless video cameras typical have a radius of about 100 feet in any direction -- the distance of about 50 average human steps or one third the length of a football field. That's at least the distance from a home to the street -- where a drive-by video spy could tap into your camera's signal.

Equipped with a more expensive signal booster antennae -- also readily available over the counter -- anyone can virtually trespass from as much as a quarter mile away.

Fishman and others advise consumers to buy more expensive closed-circuit video surveillance systems, video camera systems that use a home's existing wiring or wireless systems with proven encryption technology.

Shopping with real security in mind would also serve as a grassroots vote against unencrypted wireless systems.

"There's also the fact that people can cruise by and capture images and the person who has the camera in the home is never going to find out about it until it's too late to do anything, because the home has been burglarized. I don't like to use cordless or digital phones. I prefer hard wired because I'm familiar with the subject and I just don't want the risk. I would advise anyone to spend the extra money to get a hard-wired system or one with some degree of encryption," Fishman said.