No. 99-8508. Decided June 11, 2001

JUSTICE SCALIA delivered the opinion of the Court.

This case presents the question whether the use of a thermal-imaging device aimed at a private home from a public street to detect relative amounts of heat within the home constitutes a "search" within the meaning of the Fourth Amendment.


In 1991 Agent William Elliott of the United States Department of the Interior came to suspect that marijuana was being grown in the home belonging to petitioner Danny Kyllo, part of a triplex on Rhododendron Drive in Florence, Oregon. Indoor marijuana growth typically requires high-intensity lamps. In order to determine whether an amount of heat was emanating from petitioner's home consistent with the use of such lamps, at 3:20 a.m. on January 16, 1992, Agent Elliott and Dan Haas used an Agema Thermovision 210 thermal imager to scan the triplex. Thermal imagers detect infrared radiation, which virtually all objects emit but which is not visible to the naked eye. The imager converts radiation into images based on relative warmth-black is cool, white is hot, shades of gray connote relative differences; in thatrespect, it operates somewhat like a video camera showing heat images. The scan of Kyllo's home took only a few minutes and was performed from the passenger seat of Agent Elliott's vehicle across the street from the front of the house and also from the street in back of the house. The scan showed that the roof over the garage and a side wall of petitioner's home were relatively hot compared to the rest of the home and substantially warmer than neighboring homes in the triplex. Agent Elliott concluded that petitioner was using halide lights to grow marijuana in his house, which indeed he was. Based on tips from informants, utility bills, and the thermal imaging, a Federal Magistrate Judge issued a warrant authorizing a search of petitioner's home, and the agents found an indoor growing operation involving more than 100 plants. Petitioner was indicted on one count of manufacturing marijuana, in violation of 21 U. S. C. §841(a)(1). He unsuccessfully moved to suppress the evidence seized from his home and then entered a conditional guilty plea....


The Fourth Amendment provides that "[t]he right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated." "At the very core" of the Fourth Amendment "stands the right of a man to retreat into his own home and there be free from unreasonable governmental intrusion."  With few exceptions, the question whether a warrantless search of a home is reasonable and hence constitutional must be answered no.

On the other hand, the antecedent question of whether or not a Fourth Amendment "search" has occurred is not so simple under our precedent. The permissibility of ordinary visual surveillance of a home used to be clear because, well into the 20th century, our Fourth Amendment jurisprudence was tied to common-law trespass.  We have since decoupled violation of a person's Fourth Amendment rights from trespassory violation of his property, but the lawfulness of warrantless visual surveillance of a home has still been preserved. As we observed, "[t]he Fourth Amendment protection of the home has never been extended to require law enforcement officers to shield their eyes when passing by a home on public thoroughfares."

One might think that the new validating rationale would be that examining the portion of a house that is in plain public view, while it is a "search" despite the absence of trespass, is not an "unreasonable" one under the Fourth Amendment. But in fact we have held that visual observation is no "search" at all-perhaps in order to preserve somewhat more intact our doctrine that warrantless searches are presumptively unconstitutional.... As Justice Harlan's oftquoted concurrence described it, a Fourth Amendment search occurs when the government violates a subjective expectation of privacy that society recognizes as reasonable. We have subsequently applied this principle to hold that a Fourth Amendment search does not occur-even when the explicitly protected location of a house is concerned-unless "the individual manifested a subjective expectation of privacy in the object of the challenged search," and "society [is] willing to recognize that expectation as reasonable...."

The present case involves officers on a public street engaged in more than naked-eye surveillance of a home. We have previously reserved judgment as to how much technological enhancement of ordinary perception from such a vantage point, if any, is too much. While we upheld enhanced aerial photography of an industrial complex in Dow Chemical, we noted that we found "it important that this is not an area immediately adjacent to a private home, where privacy expectations are most heightened."


It would be foolish to contend that the degree of privacy secured to citizens by the Fourth Amendment has been entirely unaffected by the advance of technology.  The question we confront today is what limits there are upon this power of technology to shrink the realm of guaranteed privacy.

The Katz test-whether the individual has an expectation of privacy that society is prepared to recognize as reasonable-has often been criticized as circular, and hence subjective and unpredictable. While it may be difficult to refine Katz when the search of areas such as telephone booths, automobiles, or even the curtilage and uncovered portions of residences are at issue, in the case of the search of the interior of homes-the prototypical and hence most commonly litigated area of protected privacy-there is a ready criterion, with roots deep in the common law, of the minimal expectation of privacy that exists, and that is acknowledged to be reasonable. To withdraw protection of this minimum expectation would be to permit police technology to erode the privacy guaranteed by the Fourth Amendment. We think that obtaining by sense-enhancing technology any information regarding the interior of the home that could not otherwise have been obtained without physical "intrusion into a constitutionally protected area," constitutes a search-at least where (as here) the technology in question is not in general publicuse. This assures preservation of that degree of privacy against government that existed when the Fourth Amendment was adopted. On the basis of this criterion, the information obtained by the thermal imager in this case was the product of a search.

The Government maintains, however, that the thermal imaging must be upheld because it detected "only heat radiating from the external surface of the house."  The dissent makes this its leading point, see post, at 1, contending that there is a fundamental difference between what it calls "off-the-wall" observations and "through-the-wall surveillance." But just as a thermal imager captures only heat emanating from a house, so also a powerful directional microphone picks up only sound emanating from a house-and a satellite capable of scanning from many miles away would pick up only visible light emanating from a house. We rejected such a mechanical interpretation of the Fourth Amendment in Katz, where the eavesdropping device picked up onlysound waves that reached the exterior of the phone booth. Reversing that approach would leave the homeowner at the mercy of advancing technology-including imaging technology that could discern all human activity in the home. While the technology used in the present case was relatively crude, the rule we adopt must take account of more sophisticated systems that are already in use or in development..... As for the dissent's extraordinary assertion that anything learned through "an inference" cannot be a search,  that would validate even the "through-the-wall" technologies that the dissent purports to disapprove. Surely the dissent does not believe that the through-the-wall radar or ultrasound technology produces an 8-by-10 Kodak glossy that needs no analysis.

The Government also contends that the thermal imaging was constitutional because it did not "detect private activities occurring in private areas." The Fourth Amendment's protection of the home has never been tied to measurement of the quality or quantity of information obtained. In Silverman, for example, we made clear that any physical invasion of the structure of the home, "by even a fraction of an inch," was too much, and there is certainly no exception to the warrant requirement for the officer who barely cracks open thefront door and sees nothing but the nonintimate rug on the vestibule floor. In the home, our cases show, all details are intimate details, because the entire area is held safe from prying government eyes.

Limiting the prohibition of thermal imaging to "intimate details" would not only be wrong in principle; it would be impractical in application, failing to provide "a workable accommodation between the needs of law enforcement and the interests protected by the Fourth Amendment." To begin with, there is no necessary connection between the sophistication of the surveillance equipment and the "intimacy" of the details that it observes-which means that one cannot say (and the police cannot be assured) that use of the relatively crude equipment at issue here will always be lawful. The Agema Thermovision 210 might disclose, for example, at what hour each night the lady of the house takes her daily sauna and bath-a detail that many would consider "intimate"; and a much more sophisticated system might detect nothing more intimate than the fact that someone left a closet light on. We could not, in other words, develop a rule approving only that through-thewall surveillance which identifies objects no smaller than 36 by 36 inches, but would have to develop a jurisprudence specifying which home activities are "intimate" and which are not. And even when (if ever) that jurisprudence were fully developed, no police officer would be able to know in advance whether his through-the-wall surveillance picks up "intimate" details-and thus would be unable to know in advance whether it is constitutional....

We have said that the Fourth Amendment draws "a firm line at the entrance to the house." That line, we think, must be not only firm but also bright-which requires clear specification of those methods of surveillance that require a warrant. While it is certainly possible to conclude from the videotape of the thermal imaging that occurred in this case that no "significant" compromise of the homeowner's privacy has occurred, we must take the long view, from the original meaning of the Fourth Amendment forward.

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.

Since we hold the Thermovision imaging to have been an unlawful search, it will remain for the District Court to determine whether, without the evidence it provided, the search warrant issued in this case was supported by probable cause-and if not, whether there is any other basis for supporting admission of the evidence that the search pursuant to the warrant produced.

The judgment of the Court of Appeals is reversed; the case is remanded for further proceedings consistent with this opinion.


There is, in my judgment, a distinction of constitutional magnitude between "through-the-wall surveillance" that gives the observer or listener direct access to information in a private area, on the one hand, and the thought processes used to draw inferences from information in the public domain, on the other hand. The Court has crafted a rule that purports to deal with direct observations of the inside of the home, but the case before us merely involves indirect deductions from "off-the-wall" surveillance, that is, observations of the exterior of the home. Those observations were made with a fairly primitive thermal imager that gathered data exposed on the outside of petitioner's home but did not invade any constitutionally protected interest in privacy.  Moreover, I believe that the supposedly "bright-line" rule the Court has created in response toits concerns about future technological developments is unnecessary, unwise, and inconsistent with the Fourth Amendment....

APPENDIX(Images and text reproduced from defendant's exhibit 107)

Infrared image of a video frame from the videotape submitted as evidence in this case. The thermogram indicates the suspect house as it appeared with the Gain and contrast in its default setting. Only the outline of the house is visible. The camera used was the Thermovision 210. Top Right: Infrared image of a subsequent videoframe taken from the videotape. The gain and contrast settings have been increased in order to make the walls and roof of the structure appear hotter than what it actually is. Bottom Left: Infrared image of the opposite side of the suspects house. The thermogram is also taken from the same videotape. The camera settings are in the default mode and the outline of the house is barely visible. Only the hot electrical transformer and the street light are identifiable. Bottom Right: The same image, but with the gain and contrast increased. This change in camera settings cause any object to appear hotter than what it actually is. The arrow indicates the overloading of a area immediately around a hot object in this case the electrical transformer and the streetlight. This overloading of the image is a inherent design flaw in the camera itself.

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