IS USE OF THERMAL HEAT IMAGING A "SEARCH" GOVERNED BY THE FOURTH AMENDMENT? How The Supreme Court Should Resolve The Kyllo Case

On February 20, the Supreme Court heard argument in Kyllo v. United States — a case in which an Oregon man is challenging his conviction for growing marijuana in his home. He argues that the conviction should be reversed because the police searched his premises without a warrant in violation of the Fourth Amendment. (His conviction is based on a contingent guilty plea that will be voided if the Court says the warrantless intrusion by which the police discovered his crime violated the Fourth Amendment.)

Fourth Amendment-based appeals, in which a defendant challenges the legality of a search that revealed evidence used to convict him, are fairly routine. What makes this one worthy of the high court's attention is the nature of the alleged "search."

The police used a thermal imaging device to detect heat patterns generated by Kyllo's home. The device revealed that parts of the house were radiating unusually high levels of heat, of the sort created by high-intensity lights that often are (and were in this instance) used to cultivate marijuana. The central question in Kyllo is whether the use of this device constituted a "search" — and was therefore illegal, since no warrant had issued.

The government claims that no "search," for Fourth Amendment purposes, occurred here, because the device took its measurements from a public vantage point. Kyllo counters that because the thermal detector gave the police information about the inside of a person's home, using it did constitute a search.

Who is correct? To put the case in context, consider the sorts of police activity that Court precedents deem to be "searches," such that the police must either obtain a warrant before conducting these activities, or else convince a court later that a legal exception, such as the presence of exigent circumstances, justified the failure to procure a warrant.

A search is an invasion by the government of a person's "reasonable expectation of privacy." Entering a person's house, recording her telephone conversations, and opening the trunk of her car all count as searches, when carried out by the police. So do taking blood or urine samples for testing, patting down individuals' outer garments, and tearing open mail.

But not every act of police investigation is a search for purposes of the Fourth Amendment. Even without a warrant, the police may freely observe whatever the rest of the public is in a position to observe. For example, they may watch a suspect walk down the street. They can also listen to conversations that people have out in public. And they can smell (or have a drug-sniffing dog smell) odors that emanate from people or their luggage.

More controversially, the Supreme Court has held that attaching a tracking device to a person's car to reveal where (in public) the car is at all times does not constitute a search. Neither does going through a person's garbage left on the curb for pickup. Nor does monitoring (with a pen register) the telephone numbers that an individual calls from his home, information that is already monitored by the telephone company.

The theory of all of these cases is the same: Once an individual exposes what is personal to the public, the personal item loses its Fourth Amendment privacy protection. Police, in other words, are free to observe what everyone else can observe.

Heat detection technology seems, at least initially, to fit neatly within the first group of cases; it detects activity that most people cannot observe, and allows the police to learn information that other people are not equipped to learn. (After all, few, if any of us, have access to heat detection technology with which to snoop on our neighbors' houses' heat patterns!)

On the other hand, the device at issue measures only the heat that is already outside the house — not the heat within it. Just as a person who leaves her home exposes herself to public scrutiny, it might seem that once heat escapes the home, it too is available for public observation. So how is one to decide this case?

Neither the fact that most people lack heat detection technology, on the one hand, nor the fact that the detected heat is outside the house, on the other, should be dispositive. Rather than mechanically applying precedents and focusing formalistically on facts like these, we should look instead to the purposes behind the Fourth Amendment.

The real question is whether we ought to strive to preserve as private the observations that police (or others) may make on the basis of thermal detectors. Put another way, to what degree does the individual, in going into his house and shutting the door, wish to protect from exposure the heat patterns emanating from his home?

In addition, how should the Justices go about accessing the intuitions needed to answer this question?

During oral argument, the government attorney stated that no one cares about preserving the privacy of the heat outside his or her walls. This seems true, as far as it goes. As Justice Breyer responded, however, the expectation of privacy at issue is not in the emanation of heat, but in the revelation to the police, via that heat, of what is going on inside the house.

The Deputy Solicitor General replied, in turn, that the device does not tell people what is taking place in the house. Justice Souter pointed out, however, that this is not quite correct: The thermal imaging device can sometimes show people moving around in darkness. Though the device can sometimes detect human forms and activity, however, it generally cannot do so, and Kyllo concedes that it did not do so in his specific case.

What about the very fact that this device can reveal that someone may be growing marijuana in the house? Kyllo clearly did not want the police to know this piece of information. Is that desire on his part sufficient to give rise to a reasonable expectation of privacy in his case?

In attempting to resolve this question, consider a hypothetical case. Imagine that there were a machine that could give us a list of all of the places in the country concealing decomposing corpses. The machine could scan the entire United States for corpses, and corpses alone; beep a few times; and then provide a printout of an address for each place (including private residences but not cemeteries or mausoleums) in which a decomposing corpse is concealed. Does this machine invade anyone's legitimate expectation of privacy?

The person hiding a corpse in his house — let us call him "Adam" — would say "yes." But is his privacy claim any more compelling than the claim of his neighbor — call her "Eve" — who has hidden a corpse in the river and feels invaded when the police expose the contents of the river and learn that there is a corpse buried there? I believe it is not; both privacy claims are equally meritless.

If the investigating police do not learn any private facts about Adam or about his behavior other than the fact that there is a corpse concealed in his home, then in what sense has the corpse detector "searched" him any more than it has Eve? It is the incidental exposure and revelation of non-criminal information that turns the pure disclosure of crime into an invasion of privacy, a search — and no such exposure or revelation happened either in Adam's case, or in Eve's.

In analyzing whether the use of a thermal imaging device ought to count as a "search," we might ask how much this device resembles our hypothetical corpse detector. The government claims that the imaging device reveals nothing about what is going on in the house. But the Court — following Justice Souter's line of questioning — might wish to demand greater assurances. If there is a heat pattern associated with cultivating marijuana, then it is worth asking whether there are heat patterns associated with legal activities, too.

Could police use the machine to detect, for example, how many people are in the house at any given time? Could they detect whether the people are in separate rooms or together? If the technology may be desensitized — so that it reveals only marijuana plant cultivation but nothing else — it may be appropriate to allow its use, in that desensitized form alone, without a warrant.

The prospect of technology examining our homes, without our consent or even our knowledge, seems inherently frightening, and rightly so. Perhaps some day a thermal detection device could provide what is essentially a moving picture of what is occurring inside a person's home. Use of such a device would surely represent a gross invasion of privacy — and constitute a "search" that we should hesitate to permit even when police have probable cause and a warrant, and certainly should never permit when they do not.

At the same time, however, we should try not to react with irrational prejudice to an unfamiliar technology. It is important to remember that the more effectively and precisely the police can separate the guilty from the innocent, the less vulnerable we will be to the incidental burdens of police error (which can include costs not only to privacy but to freedom from invidious discrimination as well).

Probable cause and a warrant are ultimately only imperfect tools for determining where police are most likely to discover crime. If technology can perfect the tools so that they expose less that is personal, and more that is criminal, it has the potential to increase rather than decrease the privacy of us all.