Police violate the Constitution if they use a heat-sensing device to peer inside a home without a search warrant, the Supreme Court ruled Monday.

An unusual lineup of five justices voted to bolster the Fourth Amendment's protection against unreasonable searches and threw out an Oregon man's conviction for growing marijuana.

Monday's ruling reversed a lower court decision that said officers' use of a heat-sensing device was not a search of Danny Lee Kyllo's home and therefore they did not need a search warrant.

In an opinion written by Justice Antonin Scalia, by many measures the most conservative member of the court, the majority found that the heat detector allowed police to see things they otherwise could not.

"Where, as here, the government uses a device that is not in general public use to explore details of the home that would previously have been unknowable without physical intrusion, the surveillance is a 'search' and is presumptively unreasonable without a warrant," Scalia wrote.

While the court has previously approved some warrantless searches, this one did not meet tests the court has previously set, Scalia wrote.

The decision means the information police gathered with the thermal device — namely a suspicious pattern of hot spots on the home's exterior walls — cannot be used against Kyllo.

The court sent the case back to lower courts to determine whether police have enough other basis to support the search warrant that was eventually served on Kyllo, and thus whether any of the evidence inside his home can be used against him.

Justices Clarence Thomas, David H. Souter, Ruth Bader Ginsburg and Stephen Breyer joined the majority.

Justice John Paul Stevens wrote a dissenting opinion joined by Chief Justice William H. Rehnquist, and Justices Sandra Day O'Connor and Anthony M. Kennedy.

At issue was how modern police technology fits into the court's long line of decisions on what should be considered a search requiring a court warrant.

Last year, the Supreme Court ruled that police must get bus passengers' consent or a search warrant before squeezing their luggage to see if drugs might be inside. The court also requires a warrant to put a "bug" in someone's home or in a telephone booth.

But the justices have said police do not need a warrant to go through someone's garbage left on the curb, fly over a backyard to see what is on the ground, or put a beeper on a car to make it easier to follow.

Kyllo was arrested in January 1992 and charged with growing marijuana at his home in Florence, Ore.

Police had been investigating his neighbor, but they focused on him after they trained a thermal imaging device on his home and saw signs of high-intensity lights. Using those images, electricity records and an informant's tip, police got a warrant and searched Kyllo's home, finding more than 100 marijuana plants.

Kyllo contended the marijuana plants could not be used as evidence against him because the police did not have a search warrant when they used the heat-sensing device. A judge ruled against him, and Kyllo pleaded guilty on condition he could appeal the search issue.

The 9th U.S. Circuit Court of Appeals upheld the use of the device, saying it should not be considered a search.

During arguments at the Supreme Court in February, Kyllo's lawyer told the justices that people should feel free to let down their guard at home without fear of the government unreasonably looking over their shoulder.

The Justice Department contended the heat-sensing device did not intrude on Kyllo's home but instead passively detected the heat that escaped from it, and the court's dissenters apparently agreed.

Police gathered only information available on the outside walls, and used "a fairly primitive" device to do so, Stevens wrote.

Using the Thermovision device "did not invade any constitutionally protected interest in privacy," Stevens wrote.

The case is Kyllo v. U.S., 99-8508.