Can the Police Now Use Thermal Imaging Devices Without a Warrant? A Reexamination of Kyllo in Light of the Widespread Use of Infrared Temperature Sensors

In Kyllo v. United States, 533 U.S. 27 (2001), the Supreme Court held that it violated the Fourth Amendment to direct an infrared thermal imaging device at a home without a warrant to determine the home’s temperature. This post asks whether that result is still good law. I realize that probably sounds a bit nutty at first, as Kyllo is only a few years old. But Kyllo deliberately adopted a test designed to let the result change with social practice . This post asks whether changing social practices already allow the police to use thermal imaging devices without a warrant.

I’ll look at the problem in three steps. First, I’ll explain the relevant Fourth Amendment test from Kyllo. Second, I’ll explain how technology and social practice have changed in the eight-and-a-half years since the Kyllo decision. And third, I’ll put the pieces together and ask whether Kyllo‘s result remains good law. My bottom line: I’m not really sure, but there is a decent case to be made that the police can now use thermal imaging devices without a warrant consistently with Kyllo.

I. Kyllo and General Public Use

In Kyllo, the police used an infrared thermal imaging device called an “Agema Thermovision 210” to scan a suspect’s home from the city street. The scan tool a few minutes, and it revealed that the roof over Kyllo’s garage was unusually hot — a sign, the government though, that the suspect was growing marijuana under heat lamps in the garage attic. The Supreme Court announced the following rule: “when . . . the Government uses a device that is not in general public use, to explore details of the home that would previously have been unknowable without physical intrusion, the surveillance is a “search” and is presumptively unreasonable without a warrant.” Because infrared temperature sensing was not in “general public use,” the thermal imaging was a “search” that required a warrant.

There are several ways to justify the “general public use” limitation on the result in Kyllo. Perhaps the idea was that if a technology is widely used by the public, it is not longer reasonable expect that it won’t be used. This reflects what I have called the probabilistic model of Fourth Amendment protection, and is hinted at in footnote 6 of Kyllo. Alternatively, perhaps the Court was trying to square the result in Kyllo with cases going back to the 1920s that that had allowed the police to use flashlights. See United States v. Lee, 274 U.S. 559, 563 (1927); United States v. Dunn, 480 U.S. 294 (1987) . After all, flashlights are devices used to explore details previously unknowable without a physical intrusion, and yet they have always been allowed.

Whatever the reason for the limitation, it indicates that when remote infrared temperature detection devices enter general public use, the police can use them, too. Justice Scalia had no problem saying that such devices were not in general public use in 2001. In response to the dissent’s claim a hearing was required to determine how many of the devices were used, Justice Scalia responded that the majority was “quite confiden[t]” that the devices were not in general public use.

That seemed right in 2001. But what about now?

II. Common Uses of Thermal Imaging Devices a Decade After Kyllo

Fast forward a few years, and remote infrared temperature-sensing has become quite common in a wide range of applications. The most widely-used devices are “single point” thermal imagers that are widely marketed to the public as infrared thermometers. These devices are akin to temperature guns: They tell you the temperature of whatever surface you’re pointing it at, usually aided by a laser sight to tell you exactly were you’re aiming. Here’s a Black & Decker model available for $50 at Amazon (with free super saver shipping, and note the 43 glowing user reviews). I myself bought a Raytek ST20 for about $120 two or three years ago. You can see how it works in this short video:

These thermometers have entered widespread use in recent years for a wide range of applications. For example, here’s an advertisement of an infrared-sensing device marketed to moms to take their babies’ temperatures:

More often, the devices are marketed to individuals for automotive or home insulation use. The ability to pinpoint surface temperatures helps you know where you need to insulate your home better for the winter; it also lets you find short circuits or know if an engine or electrical device is dangerously overheating. These devices seem to have entered the mainstream, as I see advertisements for them in catalogs. Here’s an advertisement that gives you an idea of typical uses:

Although the the single-point devices marketed to home users are in the $50-$150 range, the full-camera models now start around $2,000. Here’s the FLIR/Extech i5 handheld thermal imager available from Amazon for $1,995. These cameras don’t just give a single point temperature reading. Instead, they give you a full picture in real-time. In effect, they take a readings from lots of individual points and assemble the readings graphically to give you a full image of the temperature. Here’s an advertisement for one of the full-camera models that is aimed at property managers and HVAC companies:

III. Is the Result in Kyllo Still Good Law?

Ok, so that’s the current state of the technology. Now let’s go back to the Fourth Amendment.

The various devices I have described can be used to do what the government did in Kyllo, namely find a “hot” portion of wall or roof on a house. I’ve tried this on my own house with my Raytek ST20, the one I bought for $120 a few years ago. I stood in front of my house in the winter, when I knew the furnace was up and running, and I pointed the device at the exterior walls of my home. By scanning up and down and side to side from outside the house, I could see the rough outlines of where inside the house the radiators were located. It turns out that the radiators create hot spots on the wall next to them that make those portions of the exterior wall around 5-10 degrees warmer than the rest.

So here’s the question. Can the police use these devices now? Are either the single-point or full-camera infrared imaging devices in “general public use”? Imagine a police officer goes on Amazon and buys the $50 Black & Decker model and directs it at a home to look for hot walls that might reveal marijuana lamps. That was a “search” in 2001. But is it a “search” in 2010?

Also, does it depend how you define the “device”? When the Supreme Court enacts special rules that regulate certain technological devices, there is always the level of generality problem in interpreting the rule. You need to figure out how broadly or narrowly to construe the “device” covered by the rule. For example, perhaps you might say that all of the single-point infrared sensors are one “device” and the full-image sensors are another, and perhaps the former is in general public use while the latter is not? (Perhaps this is arbitrary, but then maybe not: After all, the single-point devices don’t automatically generate an “image,” and therefore perhaps are not “thermal imaging devices” even if they yield the same information.) Or perhaps they should all be grouped together?

To be clear, I am not posting this to be critical of Kyllo (although I have been critical of it elsewhere for other reasons). Rather, I am just trying to figure out how the case applies a decade or so later. Kyllo deliberately adopted a rule that allows the outcome to change along with society: I’m wondering if the intervening changes in society, as seen in Part II above, mean that the devices that the case prohibited without a warrant can now be used. I can’t be sure of the answer because the courts have never elaborated on the test for “general public use.” (Just how “general” is general enough?) But taking the case seriously, it seems to me that there’s at least a plausible case that the police can now use thermal imaging devices — or at least the simple single-point infrared devices — without a warrant.

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