Kyllo v. United States

Kyllo v. United States, 533 U.S. 27 (2001), held in a 5—4 decision which crossed ideological lines that the use of a thermal imaging, or FLIR, device from a public vantage point to monitor the radiation of heat from a person's home was a "search" within the meaning of the Fourth Amendment, and thus required a warrant.
Facts.
On January 16, 1992, the United States Department of the Interior used a thermal imaging device outside of Danny Lee Kyllo's home in Florence, Oregon. According to the District Court that presided over Kyllo's evidentiary hearing, the device could not "penetrate walls or windows to reveal conversations or human activities. The device recorded only heat being emitted from the home." The device showed that there was an unusual amount of heat radiating from the roof and side walls of the garage compared with the rest of his house. (The assumption is that to grow marijuana indoors, one needs to provide a large amount of light in order for the plants to photosynthesize.) This information was subsequently used to obtain a search warrant, where federal agents discovered over 100 marijuana plants growing in Kyllo's home. Kyllo was charged with growing marijuana in his Oregon home. Kyllo first tried to suppress the evidence obtained from the thermal imaging search, but then he pleaded a conditional guilty. 
Kyllo appealed to the United States Court of Appeals for the Ninth Circuit on the grounds that observations with a thermal-imaging device constituted a search under the Fourth Amendment. After issuing and withdrawing multiple opinions, on September 9, 1999, the Ninth Circuit upheld admission of the evidence, in an opinion by Judge Michael Daly Hawkins joined by Melvin T. Brunetti, with John T. Noonan Jr. dissenting.
Kyllo then petitioned the Supreme Court for a writ of certiorari, which was granted.
Opinion of the Court.
The Supreme Court ruled 5—4 that the thermal imaging of Kyllo's home constituted a search. Since the police did not have a warrant when they used the device, which was not commonly available to the public, the search was presumptively unreasonable and therefore unconstitutional. The majority opinion argued that a person has an expectation of privacy in his or her home and therefore, the government cannot conduct unreasonable searches, even with technology that does not enter the home. Justice Scalia also discussed how future technology can invade on one's right of privacy and therefore authored the opinion so that it protected against more sophisticated surveillance equipment. As a result, Justice Scalia asserted that the difference between "off the wall" surveillance and "through the wall" surveillance was non-existent because both methods physically intruded upon the privacy of the home. Scalia created a "firm but also bright" line drawn by the Fourth Amendment at the "'entrance to the house.'" This line is meant to protect the home from all types of warrantless surveillance and is an interpretation of what he called "the long view" of the Fourth Amendment. The dissent thought this line was "unnecessary, unwise, and inconsistent with the Fourth Amendment" because according to Scalia's previous logic, this firm but bright line would be defunct as soon as the surveillance technology used went into general public use, which was still undefined.
Dissent.
In the dissent Justice John Paul Stevens argued that the use of thermal imaging does not constitute a search, which requires a warrant, because any person could detect the heat emissions. He argued that this could be done by simply feeling that some areas in or around the house are warmer than others or observing that snow was melting more quickly on certain sections of the house. Since the public could gather this information, Stevens argued, there is no need for a warrant and the use of this technique is not unconstitutional. Moreover, Stevens asserted that the use of the thermal imaging device was merely "off-the-wall" surveillance because it did not detect any "intimate" details of Kyllo's home. Finally, Stevens commented on Kyllo's trying to incorporate something as intangible, fluid and public as heat into the private sphere. He explained, "Heat waves, like aromas that are generated in a kitchen, or in a laboratory or opium den, enter the public domain if and when they leave a building."