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

Thomas D. Colbridge

Criminals quickly adopt advances in technology to pursue ill-gotten gains. Law enforcement officers should just as quickly adopt new technology as weapons against those criminals. However, unlike criminals, police officers must act within the confines of their federal and state constitutions. The use of advanced technology in the “often competitive enterprise of ferreting out crime”(1) raises concerns that individual liberties will be sacrificed in the increasingly complex war on crime. Right or wrong, the specter of Orwell’s “Big Brother” looms large in the public’s mind whenever a new crime-fighting device is unveiled. The thermal imager, or forward looking infrared device (FLIR), is an example.

Several court decisions illustrate the constitutional arguments for and against police use of thermal imagers without a search warrant. Within certain guidelines, law enforcement can use thermal imagers in compliance with the requirements of the Fourth Amendment of the U.S. Constitution.


All objects with a temperature above absolute zero emit infrared radiation. The hotter an object gets, the more infrared radiation it emits. These emissions cannot be seen with the naked eye. However, a thermal imager can detect infrared radiation emitted from an object and convert its readings into a two-dimensional, black-and-white picture.

The picture contains various shades of gray, depending upon how much infrared radiation the object is emitting. The hotter areas emit larger amounts of infrared radiation and are lighter in color; the cooler areas appear darker. The device does not measure the actual temperature of its target; it only detects the relative temperatures of different areas of the object. A thermal imager is extremely sensitive and reportedly can detect temperature variations as small as 0.1 degrees centigrade.(2) The images created by the device can be projected onto a small viewing screen or preserved on video- tape or photographs. The thermal imager is small enough to be hand- held, but often is mounted under a helicopter and flown over its target.

The technology is not new. The military has used it for years on the battlefield. Law enforcement has adopted the device only recently, using it in search and rescue operations, fugitive apprehensions, and along the border to detect drug smugglers and illegal border crossings. Moreover, thermal imagers have been particularly helpful, albeit controversial, in the detection of indoor marijuana-growing operations.(3)


The Fourth Amendment to the U.S. Constitution prohibits unreasonable searches and seizures by the government.(4) In the now famous case of Katz v. United States,(5) the Supreme Court redefined a search. Recognizing that the Fourth Amendment protects “people, not places,”(6) the Court said that a search occurs whenever the government intrudes into a person’s reasonable expectation of privacy.

Justice Harlan, concurring in Katz, formulated a useful, two-pronged test to determine when there is a reasonable expectation of privacy: 1) Does the person have a subjective or actual expectation of privacy? and 2) Is that expectation one that society is willing to accept as reasonable?(7) If the answer to both questions is “yes,” any police infringement upon that expectation is considered a search.

If the police action is a search under the Katz definition, the next question is whether the search is reasonable. The Fourth Amendment does not prohibit all searches, only unreasonable ones. The Supreme Court has made this inquiry simple. Any search made without a warrant is per se unreasonable, unless it can be justified by one of several narrowly defined exceptions to the warrant requirement.(8) The Supreme Court prefers the use of search warrants. The application for the warrant takes the issue of the existence of probable cause to search away from the investigating officer and places it before a neutral and detached magistrate, adding an additional measure of protection for the private citizen.(9)

The Supreme Court has not heard a case on whether targeting a residence with a thermal imager is a search requiring a warrant. However, the question has reached several lower federal courts and some state courts. Decisions have gone both ways. Officers using thermal imagers must understand both sides of this argument in order to avoid violating constitutional requirements.


Most of the courts have decided that targeting a building with a thermal imager is not a search under the Fourth Amendment.(10) United States v. Penny-Feeney(11) was one of the first cases to consider the matter. The court’s reasoning has influenced many other courts on the issue.

The Facts

Officers in Hawaii received information from two anonymous sources that Penny-Feeney had sold marijuana in California before moving to Hawaii and continued to do so. They told police that Penny- Feeney had grown marijuana in her home for 3 years, and plants were being harvested every 6 weeks. They provided a wealth of detail about the operation. Officers obtained a search warrant for a package sent to Penny-Feeney from California. They discovered $2,700 in cash that was detected by a narcotics dog. Officers observed Penny-Feeney pick up the package and take it to her home. Police later contacted a known informant who had visited Penny-Feeney’s residence and seen the marijuana- growing operation. He, too, described the operation in great detail. Many of the details provided by the informants were corroborated by the police.

Without a warrant, police flew over Penny-Feeney’s residence at an altitude of between 1,200 and 1,500 feet in a helicopter fitted with FLIR. The thermal image of her house showed the walls and other areas of her garage as bright white, indicating significant heat was escaping from the garage. Adjacent but similar structures did not appear the same when scanned by the FLIR. The FLIR operator said the thermal image of her house was consistent with that of a structure being used for an indoor marijuana-growing operation.

Police got a warrant to search Penny-Feeney’s residence using all of this information, including the results of the FLIR scan. During execution of the warrant, police found marijuana plants and paraphernalia used to grow marijuana indoors. Penny-Feeney and her husband were indicted. They filed a motion to suppress the evidence, alleging, among other things, that the warrantless use of the FLIR by police was an illegal search under the Fourth Amendment.

No Subjective Expectation of Privacy in “Waste Heat”

Using the Katz analysis, the district court concluded that the defendants had no subjective or actual expectation of privacy in the area scanned by the police with the FLIR. The court said that FLIR is limited to “detecting differences in temperature on the surface of the object being observed,” and “did no more than gauge and reflect the amount of heat that emanated”(12) from the defendants’ house. In other words, the FLIR registered only heat escaping from the defendants’ house. The court described this escaping heat as waste heat, or “abandoned heat,”(13) because the defendants had not tried to prevent its escape. Indeed, they used fans to vent the heat to the outside, voluntarily exposing it to the public. They never attempted “to impede its escape or exercise dominion over it.”(14)

Under these circumstances, the court concluded that the defendants did not have an actual or subjective expectation of privacy in the waste heat. Consequently, the first prong of the Katz case was not satisfied: There was no actual expectation of privacy, so there was no search under the Fourth Amendment that would require a warrant.

Expectation of Privacy Not Objectively Reasonable

The court went on to say that even if the defendants had an actual expectation of privacy in this waste heat, it is not an expectation that society is willing to accept. In other words, the defendants could not satisfy the second prong of the Katz test. The court compared heat vented to the outside to trash left for collection on a public street. The Supreme Court has said that garbage bags left for pickup generally are known to be accessible to all manner of animals and people while awaiting the trash collector. In addition, the trash is voluntarily given to trash collectors, who may handle it in any fashion they choose, including giving it to government agents. Therefore, any actual expectation that the garbage will be private is not objectively reasonable.(15) By analogy, the court reasoned that waste heat is like trash: Any expectation of privacy in waste heat is objectively unreasonable because individuals may do what they want with it once it is exposed to the public.

Sense-Enhancing Technology Not Overly Intrusive

The Penny-Feeney court did not consider the fact that waste heat can only be detected with a FLIR, and not the naked eye, to be legally significant. It relied upon several Supreme Court cases approving the warrantless use of “extrasensory, nonintrusive equipment, such as the FLIR” to investigate people and objects: United States v. Knotts(16) (a beeper placed in a container), United States v. Place(17) (use of a drug detection dog), and Smith v. Maryland(18) (use of a pen register). The court thought the analogy to the use of a drug detection dog was the strongest, citing United States v. Solis.(19) Like marijuana odor emanating from a package, Penny- Feeney expected the heat to leave the garage because she deliberately vented it to the outside. Moreover, the court concluded that the use of FLIR by police, like the use of a dog in Solis, was inoffensive because it did not embarrass the defendants or involve a search of their persons. In addition, heat and odor emanations are physical facts indicating a possible crime, not protected communications between people. Finally, the use of the helicopter to aim FLIR at Penny- Feeney’s house was deemed lawful under California v. Ciraolo(20) and Florida v. Riley(21) because the police remained in navigable airspace, and the observation was physically nonintrusive because there was no “invasion of the home or curtilage.”(22)

The Court’s Conclusion

The Penny-Feeney court concluded that the police did “no more than aim a passive infrared device at defendant’s home from an aerial vantage point for the purpose of detecting disposed waste heat on the exterior of the house. No intimate details connected with the use of the home or curtilage were observed, and there was no undue noise, no wind, dust, or threat of injury.”(23) In other words, there was no search within the meaning of the Fourth Amendment.

The majority of courts that have considered the use of thermal imagers have adopted the Penny-Feeney result but have employed slightly different analytical reasoning. One court reasoned that whether a subject takes no steps to prevent heat from escaping or actively vents it outside, both actions demonstrate a “lack of concern for the heat.”(24) Another court found that from the “balance of the evidence,” the defendant had a subjective expectation of privacy because he concealed his growing operation in an underground structure; however, the court decided his expectation was objectively unreasonable.(25) All courts upholding the warrantless use of thermal imagers have agreed on the importance of two factors: the lack of a physical intrusion into the targeted area and the scanned image’s lack of intimate detail.


The case of United States v. Cusumano(26) expresses the minority view that the warrantless use of a thermal imager by the police violates the Fourth Amendment. This case was decided by a three judge panel of the federal Tenth Circuit Court of Appeals. When the judges heard the case, the panel was vacated because they concluded that the issue of using the thermal imager did not have to be resolved in this particular case. However, the panel’s opinion remains a clear statement of the minority view.

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.

Protecting the Sanctity of the Home

The Cusumano court acknowledged the government’s right to use modern technology to fight crime, but it refused to permit the warrantless use of technology that reduces the security of people in their homes. In that regard, the court cited the case of United States v. Karo(30) to support its position. In Karo, the government placed a beeper inside a can of ether to track its movements. The beeper-laden can was taken into the defendant’s house. Agents activated the beeper, revealing that the can was stored inside the residence. The Supreme Court condemned that particular use of the beeper as a warrantless search in violation of the Fourth Amendment. Using the beeper, the agents obtained information they could not have obtained from outside the curtilage of the house: namely, that the can of ether was inside.

The Cusumano Court reasoned that the use of the thermal imager was similarly objectionable: It revealed information about activities going on inside the house that police could not obtain from outside the curtilage of the home. This revelation of a single detail about activities inside the home was sufficient to violate the Fourth Amendment. Moreover, the court did not believe that the heat signature of the defendants’ home was in plain view or knowingly exposed to the public because it is not customary for individuals seeking privacy to control their heat emissions, and privacy should not “hinge upon the insulating capacity of the walls.”(31)

The Voluntary Relinquishment Rationale

The abandoned waste analogy was equally unconvincing to the Cusumano court. The relinquishment of control over heat emissions is hardly voluntary as it is with trash. The loss of heat is governed by the laws of physics and is not an area where people usually seek control to guard their privacy. In addition, while people expect that their trash may be invaded by scavengers, they hardly expect their homes to be scanned with thermal imagers.

For similar reasons, the court dismissed the analogy to a pen register. Because the telephone company is expected to record dialed numbers, users can be said to relinquish that information to the company voluntarily. For that reason, telephone users cannot be said to have either an actual or objective expectation of privacy in their dialing information. Therefore, police use of the pen register does not infringe upon any such expectation.

Distinguishing Lawful Activities from Contraband

The dog sniff analogy offered a more precise comparison. However, the court noted that the dog sniff detects only contraband that a person may not possess legally. The thermal imager, on the other hand, identifies heat from both legal and illegal activities in the home. The court also noted that dog sniffs generally take place in public areas, such as airports or border crossings, while in this case, a private home was the target.

Many law review commentators echo the Cusumano panel view that the use of a thermal imager is a search requiring a warrant.(32) Two state Supreme Courts have agreed.(33)


It is impossible to predict when, or even if, the U.S. Supreme Court will resolve this debate. In light of the disagreement over the warrantless use of thermal imagers, the following general guidelines for police officers using thermal imagers should ensure the lawful use of this technology.

Search Warrants

Law enforcement officers should consult their legal advisors to determine whether a search warrant is required to use a thermal imager in their jurisdictions. Of course, search warrants are always preferred. Given the current debate over thermal imagery, a warrant should be obtained whenever possible before using the thermal imager.

Probable Cause

None of the cases reviewed advocate that a thermal scan alone provides probable cause for a search. Heat is generated by many different activities, both legal and illegal. The results of a thermal scan provide but one of many facts the officer must combine into the mix of probable cause to obtain a search warrant and search the scanned building.

Placement of the Thermal Imager

The courts that have upheld the warrantless use of thermal imagers have stressed that officers using the device did not physically intrude upon the area scanned. Scanners were aimed at the target by officers standing outside the curtilage of the home or business or from aircraft flying within navigable airspace above the area. Thermal scanning conducted from within the curtilage of a home or from a helicopter flying below navigable airspace is plainly more intrusive. Officers should ensure that any scan is conducted from a location where they have authority to be.

The Capability of the Thermal Imager

One of the most important factors in this ongoing debate is the nature of the information the thermal imager provides. The Supreme Court has acknowledged that police may use new technology to enforce the laws(34) but has cautioned that there are limits: “An electronic device to penetrate walls or windows so as to hear and record confidential discussions…would raise very different and far more serious questions….”(35)

Does the thermal imager penetrate walls and reveal such “intimate details”(36) so as to implicate the Fourth Amendment? One federal court has noted that infrared photographs of a mobile home revealed rafters inside and that the home appeared to be split into two rooms.(37) If FLIR devices do reveal such intimate detail, either directly or through interpretation of the images, the argument against warrantless use of the device is stronger. If, on the other hand, the device merely reveals whether one structure is emitting more heat than another, the argument against warrantless use is weaker.

Officers using thermal imagers should document clearly the capabilities of their machines. A reviewing court certainly will inquire whether the particular imager can distinguish intimate details within the targeted building. Videotapes or still photographs of any thermal images should be made and retained for the court to see. Interpretation of the thermal images should be limited to the question of whether the targeted structure is emitting substantially more infrared radiation than similar structures in the area.

Technological innovation, like thermal imaging, continually will test the meaning and limits of the Fourth Amendment of the U.S. Constitution. The U.S. Supreme Court will no doubt have to shed additional light on the issue before the debate over the use of thermal imaging will cool.


1 Johnson v. United States, 333 U.S. 10, at 14 (1948).

2 Michael L. Huskins, Marijuana Hot Spots: Infrared Imaging and the Fourth Amendment, 63 U. Chi. L. Rev. 655, at 660 (Spring 1996).

3 Growing marijuana indoors requires the use of powerful artificial lights to simulate sunlight. These powerful lights generate a great deal of heat. That heat must be expelled to avoid killing the plants.

4 U.S. Const. Amend IV reads: “The right of the people to be secure in their persons, houses, papers, and effects against unreasonable searches and seizures shall not be violated, and no Warrants shall issue but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched and the persons or things to be seized.”

5 389 U.S. 347 (1967).

6 Id. at 351.

7 Id. at 361 (J. Harlan, concurring).

8 Id. at 357.

9 333 U.S. 10, at 14.

10 See e.g., United States v. Pinson, 24 F.3d 1056 (8th Cir. 1994); United States v. Ford, 34 F.3d 992 (11th Cir. 1994); United States v. Robinson, 39 F.3d 891 (8th Cir. 1994); United States v. Myers, 46 F.3d 668 (7th Cir. 1995); United States v. Ishmael, 48 F.3d 850 (5th Cir. 1995); United States v. Robinson, 62 F.3d 1325 (11th Cir. 1995).

11 773 F.Supp. 220, (D. Hawaii 1991), aff’d on other grounds, 984 F.2d 1053 (9th Cir. 1993). On appeal, the Ninth Circuit Court of Appeals declined to address the use of FLIR because it felt police had sufficient probable cause for the search independent of FLIR readings.

12 Id. at 225.

13 Id.

14 Id.

15 California v. Greenwood, 486 U.S. 35 (1987).

16 460 U.S. 276 (1983).

17 462 U.S. 696 (1983).

18 442 U.S. 735 (1979).

19 536 F.2d 880 (9th Cir. 1976).

20 476 U.S. 207 (1986). The defendant shielded his backyard marijuana plants with two high fences. Police use of an airplane to see the plants from 1,000 feet overhead was not a search under the Fourth Amendment.

21 488 U.S. 445 (1989). The defendant grew marijuana in a greenhouse located 10 to 20 feet behind his home. The home and greenhouse were surrounded by barbed wire posted with a Do Not Enter sign. Police used a helicopter flying at 400 feet to look through holes in the roof to see the marijuana. The Court held that was not a Fourth Amendment search.

22 773 F. Supp. at 228.

23 Id. (Emphasis the court’s).

24 United States v. Robinson, 62 F.3d 1325 (11th Cir. 1995), at 1329. “…the record does not indicate that he took any action to prevent the resulting heat from being emitted into the atmosphere above his house…Robinson’s inaction regarding the heat generated from his marijuana cultivation demonstrates his lack of concern for it. Thus we conclude that Robinson has not established a subjective expectation of privacy in this heat emitted from his home.” (Emphasis the court’s).

25 48 F.3d at 857. “The device, when used in an open field, does not offend the Fourth Amendment because it is passive and nonintrusive. The sanctity of one’s home or business is undisturbed.”

26 67 F.3d 1497 (10th Cir. 1995), vacated on rehearing en banc, 83 F.3d 1247 (10th Cir. 1996).

27 Id. at 1501.

28 Id. at 1503.

29 Id. at 1505-1506.

30 468 U.S. 705 (1984).

31 67 F.3d at 1507.

32 See e.g., Michael L. Huskins, Marijuana Hot Spots: Infrared Imaging and the Fourth Amendment, 63 Chi. L. Rev. 655 (Spring 1996); James Francis Barna, Reforming the Katz Fourth Amendment “Reasonable Expectation of Privacy” Test: The Case of Infrared Surveillance of Homes, 49 Wash. U. J. Urb. & Contemp. L. 247 (Summer 1996); Michael D. O’Mara, Thermal Surveillance and the Fourth Amendment: Heating Up the War on Drugs, 100 Dick. L. Rev. 415 (Winter 1996); Jonathon Todd Laba, If You Can’t Stand the Heat, Get Out of the Drug Business: Thermal Imager, Emerging Technologies, and the Fourth Amendment, 84 Calif. L. Rev. 1437 (October, 1996).

33 State v. Young, 867 P.2d 593 (Wash. 1994) (The use of the thermal imager is a violation of both the Washington state and the federal constitutions.); State v. Siegal, 934 P.2d 176 (Mont. 1997) (Use of a thermal imager in a law enforcement context is a search under the Montana Constitution.).

34 Dow Chemical Co. v. United States, 476 U.S. 227 (1986).

35 Id. at 239.

36 Id. at 238.

37 United States v. Olson, 21 F.3d 847 (8th Cir. 1994) at 848, note 5. The court did not reach the issue of the use of the FLIR device, holding there was sufficient other evidence to support a finding of probable cause.

Law enforcement officers of other than federal jurisdiction who are interested in this article should consult their legal advisors. Some police procedures ruled permissible under federal constitutional law are of questionable legality under state law or are not permitted at all.

COPYRIGHT 1997 Federal Bureau of Investigation

COPYRIGHT 2004 Gale Group

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