Fourth Amendment Implications of Military Use of Forward
     Looking Infrared Radars Technology for Civilian Law
                        Enforcement


Forw ard L ooking In frared R adars (FL IR ) reconnaissance o f structures on private lan d s does not
   constitute a search w ithin the m eaning o f the Fourth A m endm ent.


D epartm ent o f D efen se personnel engaged in such surveillance would not be su b ject to liability
    for dam ages in a constitutional tort action.


                                                                                    March 4, 1992

                M   em orandum        O   p in io n f o r t h e   General C   o u n sel

                                  D   epartm ent o f        D   efen se


   This memorandum is in response to your request for further advice con­
cerning the use of Forward Looking Infrared Radars (“FLIR”) technology by
the Department of Defense (“DoD”) to assist civilian law enforcement agen­
cies. In a memorandum dated February 19, 1991, this Office advised that,
under existing statutory authority, DoD may assist civilian law enforcement
agencies to identify or confirm suspected illegal drug production within struc­
tures located on private property by conducting aerial reconnaissance that
uses FLIR technology.1 You subsequently requested an opinion from this
Office on the question whether FLIR surveillance of structures on private
property constitutes a “search” within the meaning of the Fourth Amendment.2
A memorandum that you have made available to us preliminarily concludes that
FLIR reconnaissance of structures on private lands does constitute such a search.3
For the reasons set forth herein, we conclude that it does not.

  1M ilitary Use o f Infrared Radars Technology to Assist Civilan Law Enforcement Agencies, 15 Op.
O.L.C. 36(1991).
  2Letter for J. Michael Luttig, Assistant Attorney General, Office o f Legal Counsel, from Terrence
O ’Donnell, General Counsel, Department of Defense (Apr. 11, 1991).
  3Memorandum for Terrence O’Donnell, Genera) Counsel, Department o f Defense, from Robert M.
Smith, Jr. (Sept. 19, 1990) (“Smith Memorandum"). Other parties to examine the issue have reached
differing conclusions. Compare Memorandum for Office of the Deputy C hief of Staff for Operations
and Plans, from Patrick J. Parrish, Assistant to the General Counsel, Department of the Arm y (Sept. 17,
1990) (FLIR surveillance is a search under Fourth Amendment) with M emorandum for Joint Chiefs o f
Staff, from Lt. Col. C.W. Hoffman, Jr., Deputy LLC (Aug. 14, 1990) (FLIR not a search) an d M em oran­
dum of Staff Judge Advocate for the Commander-in-Chief of the Pacific Command (attached to Letter
for J. Michael Luttig, Assistant Attorney General, Office of Legal Counsel, from Terrence O ’Donnell.
General Counsel, Departm ent of Defense (Nov. 21, 1990)) (same).

                                                    41
                                                    L

    Our February 19 memorandum sets forth the facts relevant to FLIR tech­
 nology, and we briefly recount them here. FLIR is a passive technology that
detects infrared radiation generated by heat-emitting objects. Infrared rays
are received by the FLIR system, electronically processed, and projected on
a screen as a visual image in the shape of the object that is emitting the heat.
The w anner the object, the brighter the image of the object appears. See
 U nited States v. Sanchez, 

, 759 (9th Cir. 1987); United States v.
Kilgus, 

, 509 (9th Cir. 1978); United States v. Penny-Feeney,

(D. Haw. 1991), a ff’d sub nom. United States v. Feeney,
984 F.2d (9th Cir. 1993).
    FLIR does not have the characteristics of an X-ray technology. We have
been informed that it cannot provide information concerning the interior of
a container or structure. It detects only heat emanating from surfaces that
are directly exposed to the FLIR system. Thus, for example, if there were
heat-producing objects within a building, FLIR could detect that more infra­
red radiation was being emitted from the building’s roof than if the building
were empty, but the system could not identify the shapes of heat-emitting
objects located within the structure. Nor could the system identify the source
o f the heat or the precise location of the heat source within the structure.
    Law enforcement agencies believe that FLIR technology can be useful in
identifying buildings that house marijuana crops, or methamphetamine or
other drug processing laboratories. In particular, FLIR can aid law enforce­
ment officials in establishing probable cause to believe that criminal activity
is being conducted within a particular building by determining whether the
building is radiating unusually large amounts of heat (due to the use of high
intensity lighting or combustion generators) or unusually small amounts of
heat (due to heavy insulation designed to mask the use of lighting or genera­
tors). Recently, therefore, federal and state law enforcement agencies have
requested that military aircraft equipped with FLIR fly over suspect build­
ings on private lands and produce infrared images of those structures.4
    We concluded in our February 19 memorandum that DoD has authority to
provide the requested assistance under the provisions of 10 U.S.C. §§ 371-378,
which are designed to promote cooperation between military personnel and ci­
vilian law enforcement officials. We now consider whether such assistance
constitutes a “search” within the meaning of the Fourth Amendment to the
Constitution.
   4 The D epartm ent of Defense has informed us of three requests for assistance that present the question
w hether such surveillance constitutes a Fourth Amendment search. The Drug Enforcement Adminis­
tration (“DEA ") has asked the Army to conduct infrared imaging o f a bam on private land in which the
DEA suspects that m arijuana is being cultivated. Second, a law enforcem ent agency has requested that
an A rm y flight crew conduct a training mission over certain private lands and buildings in the vicinity
o f W ichita, K ansas, using an Army helicopter equipped with FLIR, to identify suspected illegal mari­
ju an a cultivation. Third, the DEA has asked that the Army undertake flights in OH-58D helicopters
equipped w ith FLIR, at a height of at least 500 feet above ground, to identify dwellings and other
structures on private land in Arizona that the DEA suspects contain methamphetamine laboratories.

                                                  42
                                      II.

   The Fourth Amendment provides:

       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 particu­
       larly describing the place to be searched, and the persons or
       things to be seized.

U.S. Const, amend. IV. Until the 1960’s, the Supreme Court interpreted the
amendment to apply only to searches or seizures of the tangible things re­
ferred to in the text: “persons, houses, papers, and effects.” In O lm stead v.
United States, 

, 465 (1928), overruled by Berger v. New York,

(1967), for example, the Court held that the interception of
telephone conversations by government wiretaps did not implicate the Fourth
Amendment, reasoning that “[t]he language of the Amendment can not be
extended and expanded to include telephone wires reaching to the whole
world from the defendant’s house or office.”
   The traditional interpretation of the Fourth Amendment was also limited
to cases where the government committed a physical trespass to acquire
information. In Olmstead, the Court noted that the wiretaps were conducted
“without trespass upon any property of the 

. In
two eavesdropping cases, Goldman v. United States, 

, 134-35
(1942), overruled by Katz v. United States, 

(1967), and On Lee
v. United States, 

, 751-52 (1952), the absence of a physical
trespass was important to the Court’s conclusion that no Fourth Amendment
search had been conducted. Only where “eavesdropping was accomplished
by means of an unauthorized physical penetration into the premises occu­
pied by the petitioners” did the Court hold that eavesdropping implicated the
Fourth Amendment. Silverman v. United States, 

, 509 (1961).
   These limitations on the scope of the Fourth Amendment were eliminated
by the Court in a series of decisions during the 1960s. In 

, the Court held that “ ‘conversation’ was within the Fourth Amendment’s
protections, and that the use of electronic devices to capture it was a ‘search’
within the meaning of the Amendment” In 

, the Court
overruled the “trespass” doctrine enunciated in Olmstead, and held that eaves­
dropping conducted through the placement of a listening device on the outside
of a telephone booth constituted a “search and seizure” under the Fourth
Amendment.
   Subsequent decisions have constructed a two-part inquiry, derived from
Justice Harlan’s concurring opinion in Katz, to determine whether a govern­
ment activity constitutes a Fourth Amendment search: “[F]irst, has the

                                      43
 individual manifested a subjective expectation of privacy in the object of the
 challenged search? Second, is society willing to recognize that expectation
 as reasonable?” California v. Ciraolo, 

, 211 (1986). See also
 California u Greenwood, 

, 39 (1988); United States v. Knotts,
 

, 280-81 (1983); Smith v. M aryland, 

, 740 (1979);
 K a tz , 389 U.S. at 361 (Harlan, J„ concurring).

                                                     A.

    It will always be difficult to determine with certainty whether the owners
or users of specific structures have subjective expectations of privacy that
would be infringed by the proposed aerial reconnaissance. On the face of
the matter, however, it seems unlikely that the owner of a structure would
subjectively expect that the amount of heat emitted from the roof of the
structure will remain private. Heat is inevitably discharged from structures
that contain electrical equipment such as lights or generators, and we are
informed by DoD that FLIR equipment has been used by law enforcement
agencies for years to detect heat-emitting objects. Smith Memorandum at 6.
The only court to address the Fourth Amendment implications of FLIR con­
cluded that the owners of a private residence that was monitored by FLIR
“did not manifest an actual expectation o f privacy in the heat waste since
they voluntarily vented it outside the garage where it could be exposed to
the public and in no way attempted to impede its escape or exercise domin­
ion over it.” 

. Moreover, it is likely that
most people expect that law enforcement agencies will use information that
is available to them for the detection of crime. Absent more detailed infor­
mation about the expectations of the individuals involved, we will turn to
the second prong of the Fourth Amendment inquiry described by the Su­
preme Court for determining whether government activity constitutes a Fourth
Am endment search.5

                                                     1.

    The second question posed by the Supreme Court’s analysis is whether
FLIR surveillance, by detecting the amount of heat emitted from the exterior
o f a structure on private property, intrudes upon an expectation of privacy
that society is willing to recognize as reasonable. The Supreme Court has
not developed a clear doctrine that would indicate what is an objectively


  ’ The Suprem e Court has never relied solely on the first prong of its two-part inquiry to hold that a
governm ent activity is not a search under the Fourth Amendment. The Court itself has suggested that
the “ subjective” elem ent o f the inquiry m ay be an “ inadequate index o f Fourth Amendment protection,”
Sm ith v. M 

n.5, because, “[fjor exam ple, if the Government w ere suddenly to
announce on nationw ide television that all homes henceforth would be subject to warrantless entry,
individuals thereafter m ight not in fact entertain any actual expectation of privacy regarding their homes,
papers, and effects." 

expectation of privacy in a case where neither a physical trespass
into a home or curtilage nor a physical search of tangible objects enumer­
ated in the text of the Fourth Amendment is involved. In Rakas v. Illinois,

, 144 n.12 (1978), the Court did explain that “[l]egitimation of
expectations of privacy by law must have a source outside o f the Fourth
Amendment, either by reference to concepts of real or personal property law
or to understandings that are recognized and permitted by society.”6 Sim i­
larly, in Robbins v. California, 

, 428 (1981), disposition'bverruled
on other grounds, United States v. Ross, 

(1982), a plurality of
the Court ventured that “[expectations of privacy are established by general
social norms.” What remains unclear from these and other decisions, how­
ever, is the m ethodology that should be employed to determ ine w hat
expectations o f privacy “society” is prepared to recognize as reasonable.
    The Fourth Amendment’s protections are best discerned by reference to
the Supreme C ourt’s prior decisions in the area. Cf. Allen v. Wright, 

, 751 (1984) (given the absence of precise definitions in standing
doctrine, courts may answer standing questions through comparison with
prior cases). Applying the oft-stated principle articulated by the Court in
Katz that “[w]hat a person knowingly exposes to the public, even in his own
home or office, is not a subject of Fourth Amendment protection,” 

(citations omitted), we conclude that the use of FLIR to conduct
aerial reconnaissance of structures is not a Fourth Amendment search.7
   The Supreme Court has applied the “public exposure” rule to cases in­
volving aerial surveillance of private property.8 In Ciraolo, the Court held
that police officers did not conduct a Fourth Amendment search when they
traveled over respondent Ciraolo’s home in a fixed-wing aircraft at an alti­
tude of 1000 feet and observed, with the naked eye, marijuana plants growing
in a garden within the curtilage of Ciraolo’s home. Although the home and
garden were surrounded by double fences of six and ten feet in height, the
Court noted that “[a]ny member of the public flying in this airspace who
glanced down could have seen everything that these officers 

. Accordingly, the Court held that “respondent’s expectation
that his garden was protected from such observation is unreasonable and is
not an expectation that society is prepared to honor.” 

   Similarly, in Florida v. Riley, 

(1989), the Court held that
helicopter surveillance of the interior of a greenhouse, located within the



  ‘ The Supreme Court has referred interchangeably to “legitimate" and “reasonable” expectations o f
privacy. See 

n.4 (Powell, I., dissenting).
  ’ The District Court in 

-28, relied to some extent on the “public
exposure" doctrine to hold that FLIR surveillance of a private home did not violate a reasonable expec­
tation o f privacy of the residents. Although we concur with the result in that case, we do not agree with
all of the court's reasoning.
  8The Court has not equated the scope of the "public exposure” doctrine with subjective expectations o f
privacy. The Court has assumed that a person may have a subjective expectation of privacy even in that
which he “knowingly exposes to the public." E.g., 

.

                                                   45
curtilage of respondent Riley’s home, did not constitute a search under the
Fourth Amendment. Although the interior of Riley’s greenhouse was not
visible from the adjoining road, the investigating officer discovered that the
 sides and roof of the greenhouse were left partially open, and that the inte­
rior o f the greenhouse — including marijuana plants — could be observed
with the naked eye from a helicopter circling over Riley’s property at an
altitude o f 400 feet. A plurality of the Court, noting that “[a]ny member of
the public could legally have been flying over Riley’s property in a helicop­
ter at the altitude of 400 feet and could have observed Riley’s greenhouse,”
concluded that the case was controlled by Ciraolo. 

    Justice O ’Connor, concurring in Riley, also concluded that there was no
Fourth Amendment search, although she believed that “there is no reason to
assume that compliance with FAA regulations alone” means that the govern­
ment has not interfered with a reasonable expectation of privacy. 

(O ’Connor, J., concurring in judgment). In Justice O’Connor’s view, the
controlling question was whether “the helicopter was in the public airways
at an altitude at which members of the public travel with sufficient regular­
ity that R iley’s expectation of privacy from aerial observation was not ‘one
that society is prepared to recognize as reasonable.’” 

(quoting
K a tz , 389 U.S. at 361) (internal quotations omitted). Because Riley had not
shown that air travel at an altitude of 400 feet was extraordinary, Justice
O ’Connor concluded that the helicopter surveillance was not a “search.”
    The Court has also applied the “public exposure” doctrine to hold that an
individual has no reasonable expectation of privacy in garbage left at the
curb outside his home for pickup by trash collectors, California v. Green­
w ood, in telephone numbers dialed and thus conveyed automatically to the
telephone company, Smith v. M aryland, or in a route traveled by an automo­
bile on a public highway or the movements of objects in “open fields,” even
when they are monitored surreptitiously by an electronic beeper. United
States v. Knotts. In each of these cases, the Court reasoned that individuals
had openly displayed their activities or objects to public view and therefore
enjoyed no expectation of privacy that society is prepared to recognize as
reasonable.
    We believe that the use o f FLIR to observe heat emissions from the
exterior o f structures on private property is analogous to the surveillance
activities undertaken by the government in the “public exposure” cases. As­
suming that the aerial surveillance is to take place from airspace sometimes
used by the public — and we have not been provided with precise informa­
tion on that issue — the question presented by FLIR surveillance is quite
com parable to those decided by the Court in Ciraolo and Riley. “[T]he
home and its curtilage are not necessarily protected from inspection that
involves no physical invasion.” 

(plurality opinion).
The owner of a structure on private property knowingly, indeed almost in­
evitably, emits heat from the structure, and any member of the public flying
over the structure could detect those heat emissions with FLIR.
                                     46
   We recognize, of course, that the investigating officers in Ciraolo and
Riley conducted their visual observations with the naked eye, while FLIR
surveillance employs technology to detect what an investigator could not
observe on his own. Decisions of the Supreme Court and courts o f appeals
suggest, however, that the use of technological means to gather information
will not amount to a Fourth Amendment search where the government does
not thereby observe the interior of a structure or any other “intimate details”
o f the home or curtilage. In view of the limited information disclosed by
FLIR, we do not believe that the use of such technology in the proposed recon­
naissance missions would constitute a “search” under the Fourth Amendment.
    As a threshold matter, it is clear that the use of technological devices to
acquire information that would be unattainable through the use of natural
senses does not necessarily implicate the Fourth Amendment. In United
States v. Lee, 

, 563 (1927), the Supreme Court held that the use
of a searchlight by the Coast Guard to examine a boat on the high seas did
not violate the Fourth Amendment. The Court explained that “[s]uch use of
a searchlight is comparable to the use of a marine glass or a field glass. It is
not prohibited by the Constitution.” In On 

, the Court
said in dictum that “[t]he use of bifocals, field glasses or the telescope to
magnify the object of a witness’ vision is not a forbidden search or seizure,
even if they focus without his knowledge or consent upon what one sup­
poses to be private indiscretions.” And in United States v. White, 

, 753 (1971), a plurality of the Court concluded that “[a]n electronic
recording will many times produce a more reliable rendition of what a de­
fendant has said than will the unaided memory o f a police agent . . ., but we
are not prepared to hold that a defendant who has no constitutional right to
exclude the informer’s unaided testimony nevertheless has a Fourth Amend­
ment privilege against a more accurate version of the events in question.”
The courts of appeals have held that the interception o f communications
from radio frequencies that are accessible to the general public does not
constitute a Fourth Amendment search, even though radio waves cannot be
perceived by natural senses. E.g., United States v. Rose, 

, 26 (1st
Cir.), cert, denied, 

(1982); Edwards v. Bardwell, 

, 589 (M.D. La.), aff'd, 

(5th Cir. 1986).
    The Supreme Court discussed the use of sophisticated surveillance equip­
ment in Dow Chemical Co. v. United States, 

(1986). There,
the Court considered the Fourth Amendment implications of aerial surveil­
lance by the Environmental Protection Agency, which made use o f precise
photographic equipment to observe the open areas of an industrial facility.
In holding that the surveillance was not a “search,” the Court noted that the
photographic equipment could permit “identification of objects such as wires
as small as 1/2-inch in diameter,” 

and addressed the significance
of the equipment for the Fourth Amendment:



                                      47
                It may well be, as the Government concedes, that surveil­
           lance o f private property by using highly sophisticated
           surveillance equipment not generally available to the public,
           such as satellite technology, might be constitutionally pro­
           scribed absent a warrant. But the photographs here are not so
           revealing o f intimate details as to raise constitutional con­
           cerns. Although they undoubtedly give EPA more detailed
           information than naked-eye views, they remain limited to an
           outline of the facility’s buildings and equipment. The mere
           fact that human vision is enhanced somewhat, at least to the
           degree here, does not give rise to constitutional problems.

  So too here, FLIR does not reveal intimate details
concerning persons, objects, o r events within structures.

   The C ourt’s concern over observation of “intimate details” has been re­
peated in cases involving private homes and their curtilage. In Ciraolo, for
example, the Court went out o f its way to note that “[t]he State acknowl­
edges that ‘[a]erial observation of curtilage may become invasive, either due
to physical intrusiveness or through modem technology which discloses to
the senses those intimate associations, objects or activities otherwise imper­
ceptible to police or fellow 

n.3 (emphasis added).
M ore significantly, the plurality in Riley, in concluding that helicopter sur­
veillance of Riley’s greenhouse did not constitute a search, found it important
that “no intimate details connected with the use of the home or curtilage
were 

.9
   The courts of appeals that have considered the Fourth Amendment impli­
cations o f magnification technology used by the government to collect
information have distinguished between surveillance of the interior of a home,
which has been deemed a search, and observation of the curtilage, which has
not. In U nited States v. Taborda, 

(2d Cir. 1980), the Second
Circuit held that the use of a high-powered telescope to peer through the
window o f an apartment was a “search” under the Fourth Amendment. The
court reasoned that “[t]he vice of telescopic viewing into the interior of a
hom e is that it risks observation not only of what the householder should
realize might be seen by unenhanced viewing, but also o f intimate details o f
a p e rso n ’s private life, which he legitimately expects will not be observed
either by naked eye or enhanced vision.” 

(emphasis added).



   ’ Justice Brennan, in his dissent in Riley, criticized the majority on this point, suggesting that the police
ju st as easily could have observed intim ate details o f Riley's personal activities, although all they hap­
pened to observe was evidence of 

(Brennan, J., dissenting). FLIR, however, is
incapable of revealing intim ate details. It simply provides information about surface heat, from which
general inferences som etim es can be draw n.
A ccord United States v. Kim, 

, 1254-56 (D. Haw. 1976)
(use of telescope to view inside of apartment was Fourth Am endm ent
“search”); State v. Ward, 

, 571-73 (Haw. 1980) (use of binocu­
lars to view inside of apartment was “search”); State v. Knight, 

, 373 (Haw. 1980) (aerial observation with binoculars of inside of closed
greenhouse was “search”).10
    Subsequently, however, the Second Circuit distinguished Taborda in a
case involving the use of binoculars and a high-powered spotting scope to
observe an outdoor area adjacent to a house and garage. In United States v.
Lace, 

(2d Cir.), cert, denied, 

(1982), the court
explained that Taborda “proscribed the use of a telescope by a policeman
only so far as it enhanced his view into the interior of a home.” 

The Fourth Amendment was not implicated by the use of binoculars and a
spotting scope “in places where the defendant otherwise has exposed him­
self to public view.” 

on Taborda and Lace, the
Second Circuit declared that “it was not the enhancement of the senses p e r
se that was held unlawful in Taborda, but the warrantless invasion o f the
right to privacy in the home. In contrast, the warrantless use of supplemen­
tal resources including mechanical devices, such as binoculars, to observe
activities outside the home has been consistently approved by the courts.”
United States v. Bonfiglio, 

, 937 (2d Cir. 1983).
    The Ninth Circuit, when considering surveillance conducted with magni­
fication devices, has similarly focused on the privacy interest associated
with the area or activity observed, rather than on the nature of the technol­
ogy used. In United States v. Allen, 

(9th Cir. 1980) (Kennedy,
J.), cert, denied, 

(1981), the court held that surveillance of
private ranch property from a Coast Guard helicopter, by a Customs official
using binoculars and a telephoto lens, did not infringe upon a reasonable
expectation of privacy. The court emphasized that “[w]e are not presented
with an attempt to reduce, by the use of vision-enhancing devices or the
incidence o f aerial observation, the privacy expectation associated with the
interiors o f residences or other structures .” 

(emphasis added).
Other courts have approved the distinction for Fourth Amendment purposes
between enhanced viewing of the interior of private structures and the en­
hanced viewing of activities or objects outside such buildings. D ow Chemical
Co. v. United States, 

, 314-15 & n.2 (6th Cir. 1984), aff'd, 

(1986); United States v. Michael, 

, 258 n.16 (5th Cir.), cert,
denied, 

(1981); United States v. Devorce, 

, 201

  10 Prior to Taborda, some courts held that enhanced viewing o f the interior of certain structures on
private property did not constitute a search. Fullbright v. United States, 

, 434 (10ih Cir.)
(use o f binoculars to view inside of open shed near house), cert, denied, 

(1968); People v.
Hicks, 

, 444 (111. App. 1st Dist. 1977) (use of binoculars to view interior of residence);
State v. Thompson, 

, 513 (Neb. 1976) (use of binoculars to view interior o f residence
through curtains); State v. Manly, 

(Wash.) (use of binoculars to view interior of apartm ent),
cert, denied, 

(1975); Commonwealth v. Hernley, 

(Pa. Super. 1970) (use of
binoculars to look through window of print shop), cert denied, 

(1971).

                                                  49
 (D. Conn. 1981). Cf. New York v. Class, 

, 114 (1986) (“The
 exterior of a car, of course, is thrust into the public eye, and thus to examine it
 does not constitute a ‘search.’”); Cardwell v. Lewis, 

, 591 (1974)
 (plurality opinion) (taking of paint scrapings from the exterior of a vehicle left
 in a public parking lot did not infringe legitimate expectation of privacy where
 “nothing from the interior of the car and no personal effects, which the Fourth
 Amendment traditionally has been deemed to protect, were searched”).
    State and federal courts have followed a similar line of reasoning when
considering the use o f light-intensifying “nightscopes” to conduct surveil­
 lance in the dark. In United States v. Ward, 

, 310 (W.D.
Ark. 1982), a f f ’d in relevant p a rt, 

, 1062 (8th Cir. 1983), the
court held that the use of a nightscope to observe the movements of indi­
viduals outside a bam did not constitute a search, where “[t]he officers did
not ‘peep’ or peer into or through any windows or skylights,” or “obtain a
view o f objects or persons normally physically obscured.” 

In
 U nited States v. Hensel, 

(D. Me. 1981), a ff’d, 

(1st Cir.), cert, denied, 

(1983), the court stated that the use of
nightscopes “transgresses no Fourth Amendment rights” where drug enforce­
ment agents used the scopes to observe activities on a private dock “but
could not see into the buildings.” 

n.9. The First Circuit, al­
though not resolving the issue, subsequently characterized this conclusion as
“a reasonable position to take, given the case law on the 

. The Tennessee Court of Criminal Appeals reached the same conclu­
sion in State v. Cannon, 

(Tenn. Crim. App. 1982), noting
that the nightscope was used “to observe the traffic and activity on the
outside o f the dwelling,” but that it was “of no value in surveying activity in
the interior of the house.” 

See also Newberry v. State, 

, 549 (Fla. App. 1982), appeal dismissed, 

(Fla. 1983); State
v. D enton, 

, 584 (La. 1980). Like the Second Circuit in
Taborda with respect to telescopes, the Supreme Court of Pennsylvania has
placed limits on the use of night vision equipment when it is used to dis­
cover “intimate details” within a dwelling. In Commonwealth v. Williams,

, 966 (Pa. 1981), the Court held that when such equipment was
used for nine days to observe activity within private apartment, including
two acts of sexual intercourse, then the surveillance constituted a “search”
under the Fourth Amendment.
    Following the reasoning of these decisions, we do not believe that the use
of FLIR to detect the amount of heat emanating from structures on private
lands constitutes a Fourth Amendment search. FLIR does not permit obser­
vation of the interior of homes or other structures. It cannot be used to peer
through windows or skylights. It does not reveal even the shape or precise
location of heat-emitting objects within a building, but shows only the amount
of heat emitted from the exterior of a structure. When compared with the
observations made by investigating officers in Ciraolo and Riley (which in­
cluded the interior o f Riley’s greenhouse and the specific plants growing in
                                        50
Ciraolo’s garden) and in Lace, Allen, and other lower court decisions (which
included the movement of persons and vehicles within the curtilage o f a
residence), external heat emissions are not the sort of “intimate detail” likely
to raise concerns under the public exposure cases.11

                                                    III.

                                                     A.

   The Smith Memorandum predicts, however, that the public exposure ra­
tionale “is unlikely to be adopted by the courts” with respect to FLIR. Smith
Memorandum at 27. In its view, the public exposure doctrine should not be
extended to cases where the technology adds a “sixth sense” to those natu­
rally possessed by investigating officers. The memorandum contends that
nightscopes and binoculars reveal activities that would have been visible to
the human eye absent darkness or distance, and that the electronic beeper
employed to monitor a vehicle on public roads in United States v. Knotts
revealed only activities that would have been visible to passersby. By contrast,
the memorandum argues, FLIR “permits observation of something that pass­
ersby cannot perceive with their natural senses,” and its use is thus not likely to
be sanctioned under the public exposure doctrine. Smith Memorandum at 27.
    Assuming that there is a viable distinction between technologies that en­
hance existing senses and those that permit “extra-sensory” perception, and
assuming that FLIR permits government agents to observe what they could
not detect with their natural senses, those facts alone do not mean that the
use of FLIR is a “search” under the Fourth Amendment. Federal and state
courts have held that the interception of radio waves — which themselves
cannot be perceived by the natural senses — does not constitute a “search.”
The United States Court of Appeals for the First Circuit, for example, con­
cluded that there is no reasonable expectation of privacy in a communications
broadcast on a ham radio frequency, which is “commonly known to be a
means of communication to which large numbers of people have access as
receivers.” 

. Similarly, the Fifth Circuit summarily
affirmed the decision of a district court which concluded that “[t]here is no
reasonable expectation of privacy in a communication which is broadcast by
radio in all directions to be overheard by countless people who have pur­
chased and daily use receiving devices such as a ‘bearcat’ scanner or who

   "T h e relatively minimal information disclosed by FLIR clearly distinguishes it from X-ray-like tech­
nologies, which could perm it the viewing of persons or objects through opaque structures or containers.
As the United States said in its brief in Dow Chemical, if “the government possessed a sophisticated X-
ray device that enabled it to see through the walls of a house, there seems little doubt that the use o f such
a device to discover objects or activities located inside a dwelling would be subject to Fourth Am end­
ment regulation." B rief for the United States at 24 n.12, 

(1986) (No. 84-1259). See
United States v. Haynie, 

(4th Cir. 1980) (use of X-ray machine to reveal shapes of objects
is a Fourth Amendment search), cert, denied, 451 U.S. 972(1981); United States v. Henry, 

(9th cir. 1980) (same).

                                                     51
happen to have another mobile radio telephone tuned to the same frequency.”
E dw ards v. 

. Accord Edwards u State Farm
Ins. Co., 

, 538-39 (5th Cir. 1987). Three other circuits have
 likewise held that there is no reasonable expectation of privacy in radio
 telephone or cordless telephone conversations. Tyler v. Berodt, 

, 706-07 (8th Cir. 1989), cert, denied, 

(1990); United
States v. H all, 

, 198 (9th Cir. 1973); United States v. Hoffa, 

, 1247 (7th Cir. 1970), cert, denied, 

(1971).12 These
decisions demonstrate that the question whether the acquisition of informa­
tion is a “search” must depend on more than whether the information may
be perceived by the natural senses.
    In any event, we believe it is virtually impossible to divide surveillance
techniques neatly between those that allow “extra-sensory” perception and
those that merely employ the natural senses. It is hardly clear, for example,
that night vision equipment, the use of which has been held not to constitute
a search, permits merely “enhancement o f the natural sense of sight.” Smith
Memorandum at 27. One jurist to consider the question thought not, and
observed that a nightscope “not only magnifies what the viewer could see
with the naked eye, but also makes possible the observation of activities
which the viewer could not see because o f darkness.” State v. 

. On the other hand, the First Circuit has opined that “[u]se o f a
beeper to monitor a vehicle involves something more” than “magnification
o f the observer’s senses,” United States v. Moore, 

, 112 (1st Cir.
 1977), cert, denied, 

(1978), even though the Smith Memoran­
dum maintains that beeper surveillance reveals only activities that would be
visible to passersby, and thus is not “extra-sensory.” Smith Memorandum at
27. In short, virtually all of the devices used by investigating officers in
some sense permit the collection of information that could not “naturally”
be observed. The distinction between natural and “extra-sensory” observa­
tions thus seems to have little analytical or constitutional significance.
    Even if that distinction were important, it is not at all clear that FLIR
would be categorized properly as a device that permits observations that
humans could not make with their natural senses. At some level, heat ema­
nations can be observed through the natural sense o f sight. The naked eye
can perceive heat waves rising from a warm object. The relative speeds at
which snow melts from the roofs of various structures can give indications
about the relative heat emissions from those structures. The natural senses
can also feel heat emanations when they are in close proximity to the human
body. Thus, it could be argued that FLIR merely enhances the capacity of
the natural senses to perceive heat.
  ,2See also Slate v. Delaurier, 488 A .2d 688, 694 (R.I. 1985) (owners of cordless telephone had no
reasonable expectation o f privacy in their conversations, which could be intercepted with standard AM/
FM radio); State v. Howard, 

, 206 (Kan. 1984) (same); People v. M edina, 

, 262 (Cal. Ct. App.) (no reasonable expectation of privacy in message sent through pager system,
w here conversation “'could be intercepted by anyone with a radio scanner or another pager”), cert,
denied, 

(1987).

                                                  52
    Courts generally have held that the relevant question for determining
whether surveillance infringes upon a legitimate expectation of privacy is
not merely how information is collected but what information is collected.
If an object of government surveillance is recognized by society as enjoying
a privacy interest of sufficient magnitude, the government’s activity will
constitute a “search.” Technology that allows the government to view the
interior of a home almost certainly implicates the Fourth Amendment. But
we are not prepared to say, as the Smith Memorandum suggests, that any
“extra-sensory” technological development that assists authorities in ferret­
ing out crime is automatically one that society would deem unreasonably
intrusive, no matter how minimal the intrusion on the privacy interests of the
citizenry. The Supreme Court has “never equated police efficiency with
unconstitutionality,” Knotts , 460 U.S. at 284, and we fear that acceptance of
the Smith Memorandum’s analysis would come perilously close to doing so.

                                     B.

    More fundamentally, the Smith Memorandum suggests that extension of
the public exposure doctrine to endorse the use of FLIR would threaten to
“repudiate” Katz, because “any member of the public who could obtain a
sophisticated listening device could have heard everything the police heard”
in Katz. Smith Memorandum at 28. This contention does illustrate that the
public exposure doctrine must have limits, and it points to an internal ten­
sion in the reasoning of Katz itself. It could reasonably be argued that Katz,
given the availability of listening devices, knowingly exposed his conversa­
tions to the public by using a public telephone booth to place his calls. It
may well be that the Supreme Court will eventually be forced to revisit its
Fourth Amendment jurisprudence and explain the relationship between K atz
and the “public exposure” doctrine.
    In the light of decisions subsequent to Katz, however, it appears that the
Court concluded that the eavesdropping in Katz was a search not simply
because the FBI employed technology, but because the technology permitted
the interception of “private 

. Private com­
munications, like private papers and the interior of a home, implicate a
privacy interest of the highest degree. As Justice Brandeis explained in his
prescient dissent in Olmstead, the Supreme Court has long held that private
letters are protected by the Fourth Amendment, see Ex parte Jackson, 

(1877), and “[t]here is, in essence, no difference between the sealed
letter and the private telephone message.” Olm 

(Brandeis, J., dissenting). “Society” is plainly prepared to recognize as
reasonable the expectation that private telephone calls will remain free from
monitoring by the government. Heat emissions from the exterior of a struc­
ture — providing, as they do, no precise details about a structure’s interior
— do not, in our view, enjoy a similar status.


                                     53
                                      c.
   The principal case relied on by the Smith Memorandum for the conclu­
sion that FLIR surveillance is a Fourth Amendment search is United States
v. Karo, 

(1984). In Karo, drug enforcement agents installed an
electronic beeper in a can of ether, which they believed was to be delivered
to buyers for use in extracting cocaine from clothing that had been imported
into the United States. After the ether was delivered to the buyers, who had
no knowledge o f the presence of the beeper, the agents monitored the move­
ment of the can of ether within a private residence where it was stored and
used.
   The Court held that “the monitoring of a beeper in a private residence, a
location not open to visual surveillance, violates the Fourth Amendment
rights o f those who have a justifiable interest in the privacy of the resi­
dence.” 

After reciting the basic rule that “[s]earches and seizures
inside a home without a warrant are presumptively unreasonable absent exi­
gent circumstances,” 

the Court explained that monitoring of
the beeper inside the private residence was the functional equivalent of a
physical search of the premises:

            In this case, had a DEA agent thought it useful to enter
       the . . . residence to verify that the ether was actually in the
       house and had he done so surreptitiously and without a war­
       rant, there is little doubt that he would have engaged in an
       unreasonable search within the meaning of the Fourth Amend­
       ment. For purposes of the Amendment, the result is the same
       where, without a warrant, the Government surreptitiously em­
       ploys an electronic device to obtain information that it could
       not have obtained by observation from outside the curtilage of
       the house. The beeper tells the agent that a particular article
       is actually located at a particular time in the private residence
       and is in the possession of the person or persons whose resi­
       dence is being watched.

The Court distinguished its earlier decision in United States v.
K notts, which held that the monitoring of a beeper on public roads was not a
Fourth Amendment search. The Karo Court concluded that although the use
o f a beeper inside a home is “less intrusive than a full-scale search,” it
“reveal [s] a critical fact about the interior of the premises that the Govern­
ment is extremely interested in knowing and that it could not have otherwise
obtained without a warrant.” 

Memorandum states that it “appears likely” that the Supreme
Court would hold, primarily on the authority of Karo, that FLIR surveillance
is a Fourth Amendment search. Smith Memorandum at 25. The Memorandum

                                      54
reasons that FLIR would enable investigators to deduce whether an object,
such as a generator, is within a private structure in which there is a reason­
able expectation of privacy. Accordingly, like a beeper, FLIR could permit
the government to leam “a critical fact about the interior of the premises”
without obtaining a warrant.
    We do not believe that Karo should be read so broadly. First, it is clear
that not every acquisition of information by the government from which it
can deduce facts about the interior of a residence or other private structure
constitutes a search. In California v. Greenwood, 

(1988), for
example, the Court held that a search of trash placed outside a home for
removal by the trash collector did not infringe upon a legitimate expectation
of privacy of the homeowner. The Court reached this conclusion despite the
fact that, as the dissent pointed out, “a sealed trash bag harbors telling evi­
dence of the ‘intimate activity associated with the sanctity of a man’s home
and the privacies of life.’” 

(Brennan, J., dissenting) (internal quota­
tions omitted). Likewise, in Smith v. Maryland, 

(1979), the
Court held that the installation and use of a pen register to record telephone
numbers dialed by Smith was not a search, although the pen register re­
vealed to police telephone numbers that Smith dialed within the privacy of
his own home. See 

   Many other observations permit police to discern what might in some
cases be “critical facts” about the interior of a residence, although they
almost certainly do not constitute searches under the Fourth Amendment.
The sighting through a nightscope of smoke emanating from a chimney on
top of a house, for example, allows an inference that a fire is burning inside
the house. Observation through binoculars of light beams coming from a
window permits the conclusion that someone (or some device) has activated
a light inside the house. Yet in light of the decisions of the Supreme Court
in Ciraolo and Riley and of the various state and lower federal courts involv­
ing binoculars and nightscopes, we believe it quite unlikely that the Supreme
Court would hold, by analogy to Karo, that such observations of activity
exposed to public view infringe upon Fourth Amendment rights.
   Second, the Court in Karo rested its holding on the fact that the govern­
ment had “surreptitiously employ[ed] an electronic device to obtain
information that it could not have obtained by observation from outside the
curtilage of the 

(emphasis added). By contrast, the
owner of a structure on private property has full knowledge that heat is
emitted from the structure and, presumably, that it can be monitored by
infrared radars.13 The result in Karo would likely have been different had
the owner of the residence knowingly placed his own beeper in the ether

  11 We are informed by DoD that “infrared technology has been in use by local, state, and federal law
enforcem ent officials for years." Smith Memorandum at 6. FLIR is mentioned in a reported court
decision as early as 1977, see U nited States v. P otter, 

, 906 n.7 (9th Cir. 1977), and it has
been discussed by several courts in the last fourteen years. The existence and usefulness o f FLIR may be
                                               Continued

                                                  55
container and voluntarily conveyed the signal to anyone in the public who
might desire to monitor it. C f United States v. 

(no
reasonable expectation of privacy in communications broadcast on a ham
radio frequency); Edwards v. 

.
   The Smith Memorandum contends that “the only constitutional signifi­
cance of the fact in Karo that the beeper monitoring was done ‘surreptitiously’
appears to be that it was done without the knowledge and consent of Karo”
and that “[t]o this extent, the proposed use of FLIR is as surreptitious as was
the use of the beeper in Karo.” Smith Memorandum at 25. As noted, we
believe this analysis fails to recognize the distinction between knowing and
unknowing conveyance of information for receipt by the public. Karo did
not know that the beeper was emitting its signal from the interior of his
residence, because DEA agents surreptitiously planted the beeper in his home.
By contrast, the owner of a structure on private property knows that he is
emitting heat through the roof of the structure.14

                                                          D.

   Finally, the Smith Memorandum predicts that a court considering the use
of FLIR over private property would invoke the Supreme Court’s cautionary
note in Dow Chemical that “surveillance of private property by using highly
sophisticated surveillance equipment not generally available to the public,
such as satellite technology, might be constitutionally proscribed absent a

. See Smith Memorandum at 27. Whatever the
significance of this dictum, we do not believe it applicable to aerial recon­
naissance that makes use of FLIR. While FLIR equipment may be expensive,
we are informed that it is available to any member of the public who might
wish to purchase it for use. FLIR does not, therefore, constitute “surveil­
lance equipment not generally available to the public.”
   To be sure, the proposed uses of FLIR raise difficult Fourth Amendment
issues. FLIR enables the government to acquire information concerning
heat emissions from private structures that has not been readily available in
the past. We do not believe, however, that every technological advance in
the service of law enforcement will inevitably infringe upon expectations of
privacy that society is willing to honor. FLIR collects information about
heat that is emanating from the exterior of structures and conveyed openly
into the atmosphere. It does not reveal any precise or intimate details about

   15(....continued)
known am ong the citizenry as well, for law enforcement officials have informed DoD that individuals
attem pting to cultivate illegal drugs “w ill often insulate their growing houses in an effort to preclude
discovery of the intense heat generated by [the cultivation] process.” Smith Memorandum at 1.
   14The M em orandum also relies on a num ber of lower court decisions holding that the use o f a m agne­
tom eter to detect metal on a person is a search under the Fourth Amendment. See, e.g.. U n ited States
v. A lb a ra d o , 

(2d Cir. 1974); U nited States v. B e ll , 

(2d Cir.), cert, d e n ie d , 

(1972); U n ite d S ta te s v. E p p erso n , 

(4th Cir.), cert, d en ied , 

(1972).
                                                      Continued

                                                          56
the interior of a structure. Any member of the public flying over a building with
FLIR could acquire the information proposed to be collected by DoD personnel.
   In view of these factors and the relevant court precedents, we believe that
the proposed use of FLIR to conduct aerial reconnaissance over structures
located on private lands would not constitute a “search” under the Fourth
Amendment, unless travel at the altitude to be flown by the aircraft carrying
FLIR equipment is extraordinary. We believe this caveat is necessary, be­
cause Justice O’Connor’s concurring opinion in Florida v. Riley seemed to
indicate that aerial surveillance from airspace that is rarely, if ever, traveled
by the public would interfere with a reasonable expectation of 

(O’Connor, J., concurring); see also United States v. Hendrickson,

, 323 (8th Cir.), cert, denied, 

(1991). It is uncer­
tain whether the Supreme Court will ultimately adopt the reasoning of the
Riley plurality or Justice O’Connor concurrence, but for the time being, the
law is unsettled with respect to aerial surveillance conducted from airspace
that an individual could prove is rarely, if ever, used by the general public.
If DoD encounters a situation in which FLIR surveillance would be carried
out from airspace that is rarely used by the public, we would be pleased to
examine that issue in more depth.

                                                    IV.

    You have also expressed concern that DoD personnel who conduct FLIR
surveillance might be subject to tort liability in an action brought under
Bivens v. Six Unknown Named Agents o f Federal Bureau o f Narcotics, 

(1971). We do not believe that DoD personnel engaged in such
activity will be liable for damages. If, as we believe, FLIR surveillance
does not constitute a Fourth Amendment search, there would of course be no
constitutional violation and no potential liability.
    Even if a court were to disagree with our conclusion and hold that FLIR
surveillance is a search, we do not believe that DoD personnel would be
subject to liability for monetary damages. Federal officers are entitled to
“qualified immunity” from tort suits for actions taken in the course of their
official duties. E.g., Mitchell v. Forsyth, 

, 528 (1985); Harlow
v. Fitzgerald, 

, 819 (1982). In Anderson v. Creighton, 

(1987), the Supreme Court explained that an officer is entitled to such immunity
unless he violates a constitutional right that is “clearly established” at the time

  14(....continued)
S e e Smith M emorandum at 14 & n.33. These decisions contain little or no analysis o f the question
whether use o f such a device is a Fourth Amendment search, and we agree with DoD that “we cannot be
certain that the [Supreme] Court would agree their use is a search or that it would apply the same
analysis to use o f FLIR.” 

event, the use of a magnetometer is distinguishable from FLIR in
at least one crucial respect. The magnetometer cases do not fall within the public exposure doctrine,
because it is not true that “any member o f the public” could learn what the governm ent discovers
through a magnetometer. The government is able to make use of a magnetometer only because it can
require individuals to pass through the mechanism in order to travel on airplanes. S e e A lb a ra d o , 495
F.2d at 806-07.

                                                    57
of the officer’s action. The right must be “clearly established” in this particular­
ized sense: “The contours of the right must be sufficiently clear that a reasonable
official would understand that what he is doing violates that right.” 

    Given the uncertainty surrounding what expectations of privacy “society
is prepared to recognize as reasonable,” we do not believe that the use of
FLIR from airspace that is used by the general public — even if ultimately
held to be a Fourth Amendment search — would violate a “clearly estab­
lished” constitutional right of the owners of structures on private lands. As
our legal analysis (and the difference of opinion among those to have exam­
ined the issue) shows, a reasonable officer certainly could believe that the
use of FLIR to conduct aerial reconnaissance of private structures is lawful.
Accordingly, we do not think that DoD personnel providing that type of
assistance to civilian law enforcement agencies would be subject to liability
for damages in a constitutional tort action.

                                              TIMOTHY E. FLANIGAN
                                          Acting Assistant Attorney General
                                               Office of Legal Counsel




                                        58