Thermal imaging: much heat but little light - legal aspects of the thermal imager

FBI Law Enforcement Bulletin,The, Dec, 1997 by Thomas D. Colbridge

The Facts

In their affidavit for a search warrant, police in Cheyenne, Wyoming, included, among many other facts, the results of a warrantless thermal scan of the defendant's home and attached garage. The thermal image showed a large hot spot on one wall of the garage and a number of hot spots along the roof and near the front door of the house.

When police executed their warrant, they found an indoor marijuana-growing operation in the basement. The defendants were indicted and convicted of manufacturing marijuana. On appeal, they contended that the warrantless use of the thermal imager violated their Fourth Amendment rights. A panel of the Tenth U.S. Circuit Court of Appeals agreed.

Heat Reveals Private Activity in the Home

Using the Katz analysis, the court concluded that the defendants had an actual expectation of privacy. This court, however, framed its inquiry quite differently. The court based its analytical framework on the private activities within the house that generated the heat rather than merely focused on the heat escaping from the house.

The court said heat emitted outside the home and measured by the thermal imager is directly related to, and a function of, activities going on in the home. Viewed from that perspective, the court reasoned that the thermal imager actually created a "heat signature"(27) capable of revealing information about heat-generating activities going on inside the house. In other words, the imager painted a picture that police could translate into information regarding what the defendants were doing inside the building.

According to the Cusumano court, the question to ask is not whether the defendants expected the escaping heat to remain private, but whether they expected the indoor activities that the heat signature revealed to remain private. Clearly the defendants did, the court concluded, because they had hidden their operation in the basement and blocked the windows.

Obviously, the defendants did not take all possible steps to protect their operation from a thermal scan, but the court concluded that they should not have to anticipate and guard against "every investigative tool in the government's arsenal"(28) in order to claim an actual expectation of privacy. Otherwise, the court said, the public would be at the mercy of advances in government technology, drawing citizens into a lopsided game of "hide and seek played by the government and the people."(29)

Protecting the Privacy of Activities Within the Home Is Objectively Reasonable

The court then turned to Katz' second prong, whether the defendants' expectation of privacy was objectively reasonable. The court said the expectation that activities within someone's home would remain private is objectively reasonable because society still believes that activities carried on in the home, and not knowingly exposed to the public, should remain private. The defendants had met the second prong of the Katz test, as well. Consequently, the defendants had both an actual and an objectively reasonable expectation of privacy in the activities within their home. Therefore, the Court decided that this use of the thermal imager was a search, and the police should have obtained a warrant before scanning the home with a thermal imager.