June 11, 2001 -- Federal agents could not see inside Danny Lee Kyllo's home. Nor did they have a search warrant to enter the premises.

But they did have an infrared camera that used thermal imaging technology, enabling them to identify suspected heat lamps growing 100 marijuana plants.

They used the images to get a warrant, leading to Kyllo's arrest and conviction.

The technology, originally designed for the military, displays objects by distinguishing differences in temperature of surrounding objects, so that a person, warmer than the surrounding air, appears a different color than the air.

The Supreme Court today, in Kyllo vs. U.S., ruled that authorities scanning a home with an infrared camera without a warrant constituted an unreasonable search barred by the Fourth Amendment.

It did so, the court said, because the device is not in general use by the public, so Kyllo had an expectation of privacy, and because the imaging provided by the camera revealed details about Kyllo's home "that would previously have been unknowable without physical intrusion."

Privacy in the Home

The 5-4 opinion, written by Justice Antonin Scalia, restated the court's previous findings that a visual search of a home, without entry, is not a search and not restricted by the Fourth Amendment.

But in adapting a principle first enunciated in the landmark Supreme Court case Katz vs. United States, a case involving remote eavesdropping of someone inside a phone booth, it found a warrant is required when the person has a "subjective expectation of privacy." That expectation existed for Kyllo because he was in his own home, an idea with deep roots in common law, the ruling found.

"To withdraw protection of this minimum expectation would be to permit police technology to erode the privacy guaranteed by the Fourth Amendment," it said.

Kyllo's attorney, Kenneth Lerner, made a similar argument in court papers: "Technology that exploits invisible, sub-sensory phenomena ultimately fails to respect the traditional boundaries of society, and therefore leaves the population defenseless against such surveillance."

No Physical Intrusion

"I think this is an important ruling because a lot of people, including myself, were concerned that the court would just say the technology is fine as long as you're detecting something outside [of the home]," such as heat emissions, said Sherry Colb, a professor at Rutgers Law School.

"That would be a great cause for concern," she said, referring to a footnote in the ruling that observes there are technologies being developed that might allow authorities to see through an opaque wall.

The federal attorneys arguing the case reasoned the technology did not enable authorities to see within Kyllo's home, but, rather, to detect differences in heat on the roof and walls of his home, and so did not actually look at what was inside. A warrant was obtained, based upon that information, which led to Kyllo's arrest, guilty plea, and sentence of 63 months in prison.

But the court said the Fourth Amendment was applicable since the search provided information regarding the home's interior that otherwise could not have been obtained without a physical intrusion.

"Such a mechanical interpretation of the Fourth Amendment was rejected in Katz, where the eavesdropping device in question picked up only sound waves that reached the exterior of the phone booth to which it was attached," the court said.

Colb argues, however, that by emphasizing the traditional sanctity of the home, the court's decision in Kyllo leaves open a question about whether it applies to searches of other areas with some Fourth Amendment protection, such as the trunk of a car or a briefcase.

She notes the court found in United States vs. Place that dogs sniffing for narcotics to detect its presence in a container without opening the container was not a search defined by the Fourth Amendment.

"The majority focuses a lot on whether you are revealing content of what's inside something that would otherwise not be visible … So the dissent rightly points out that the dog sniff case is at odds with that principle," she says.

No Practical Impact?

Detective Larry Wilson of the Plano, Texas, police force, said it has been common for police to use thermal imaging on houses without first obtaining a warrant, and that will change.

But he says the police in his department and others he's trained around the country have been instructed not to use the devices without having first obtained probable cause through other means. So he says the ruling should not greatly affect current police use of infrared cameras on homes.

"Whenever we're doing an indoor grow operation investigation we've already established the necessary probable cause prior to doing the thermal imaging," said Wilson. "Now the only step that's going to be added is to get an affidavit and get a judge to do that and issue a warrant."

Thermal imaging is not precise enough itself to provide probable cause, he added.

"The way that the imager is utilized is left open to interpretation," he said. "You can't say that the heat you're looking at that's being emitted from the house through vents or whatever for sure that it's being produced from once source or another."

Kyllo's case has been remanded back to the district court where it was heard to decide whether the subsequent warranted search of Kyllo's home was justified by probable cause.