Why did Antonin Scalia find thermal imaging of a home unconstitutional in Kyllo v. United States?

In Kyllo v. United States (2001), Justice Antonin Scalia authored the majority opinion finding that the use of thermal-imaging technology to monitor the interior of a home constituted an unconstitutional search under the Fourth Amendment.1

Consistent with his commitment to originalism and textualism, Scalia’s reasoning focused on protecting the sanctity of the home, a principle deeply rooted in the historical understanding of the Fourth Amendment. He argued that when the government uses a device that is not in general public use to explore details of a home that would previously have been unknowable without physical intrusion, the surveillance is a "search" and is presumptively unreasonable without a warrant.21

By ruling this way, Scalia sought to ensure that technological advancements did not erode the core protections afforded to citizens against government intrusion into their private residences, maintaining the "bright line" of privacy that he believed the Constitution was originally intended to secure.1