Police in New York City are trying to develop something like x-ray vision, raising questions about the legal expectation for privacy in public places.

The NYPD recently announced that it's working on a mobile scanner designed to detect concealed weapons on people from up to 75 feet away. The department won't say when the devices might hit the street or how much they will cost, but Police Commissioner Raymond W. Kelly has already praised them as "a cutting-edge effort to deter gun violence."

The police department is developing the device with the U.S. Department of Defense. The device measures terahertz waves—which are naturally emitted by people and objects and pass through materials such as clothes, which is how the scanner can reveal guns or other hidden objects. The current model can scan from up to about 15 feet away; Kelly hopes to increase the range to 75 feet before mounting the device on police vehicles.

City officials tout the technology as an alternative to NYPD's controversial "stop and frisk" technique, in which officers stop people on the street for questioning—often patting them down in the process. Opponents of the practice say it unfairly targets minorities. Blacks and Hispanics accounted for 87 percent of the roughly 600,000 people stopped by the NYPD in 2010, according to the Center for Constitutional Rights, a civil rights organization based in New York City.

"I think this is a positive development," says Richard Cardinale, a New York attorney who represents plaintiffs in police misconduct cases. "The biggest benefit of stop and frisk is that police are getting guns off the street and saving lives. If you have this new way to detect guns, people don't have to suffer the indignity of being searched for no reason."

Not everyone is ready to embrace the new technology, however. "There are serious Fourth Amendment problems raised by the use of this technology," says Michael Price, counsel for the Brennan Center for Justice's Liberty and National Security Program. "The idea that police could mount a scanner on a car and take images of everyone within a certain range is troubling."

The Fourth Amendment guarantees individuals the right to be free from "unreasonable searches and seizures." Generally, the law allows cops to stop and frisk a person if the officer has "reasonable suspicion" that the person is committing or about to commit a crime. The threshold is lower than the "probable cause" required for police to search a person's car, get a warrant to search a home, or make an arrest.