Cite as: 533 U. S. 27 (2001)
Opinion of the Court
information obtained by the thermal imager in this case was the product of a search.2
The Government maintains, however, that the thermal imaging must be upheld because it detected "only heat radiating from the external surface of the house," Brief for United States 26. The dissent makes this its leading point, see post, at 41, 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 only sound 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
2 The dissent's repeated assertion that the thermal imaging did not obtain information regarding the interior of the home, post, at 43, 44 (opinion of Stevens, J.), is simply inaccurate. A thermal imager reveals the relative heat of various rooms in the home. The dissent may not find that information particularly private or important, see post, at 43-44, 45, 49- 50, but there is no basis for saying it is not information regarding the interior of the home. The dissent's comparison of the thermal imaging to various circumstances in which outside observers might be able to perceive, without technology, the heat of the home—for example, by observing snowmelt on the roof, post, at 43—is quite irrelevant. The fact that equivalent information could sometimes be obtained by other means does not make lawful the use of means that violate the Fourth Amendment. The police might, for example, learn how many people are in a particular house by setting up year-round surveillance; but that does not make breaking and entering to find out the same information lawful. In any event, on the night of January 16, 1992, no outside observer could have discerned the relative heat of Kyllo's home without thermal imaging.
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