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Thermal Imagers on Patrol

Welcome to a new column that will focus on using thermal imagers (TIs) to enhance police operations, especially patrol work. This technology can be employed in practical ways on the street to make your job easier and safer--in applications ranging from drug activity surveillance to evidence retrieval.

As an author, I am faced with a dilemma. I want to entice you into following this column. Yet, this is a technology that few have seen and fewer have used. Before looking at "street-smart" applications for a thermal imager, you must understand the basic technology. While reading about the technical aspects of an unfamiliar device can be as exciting as reading a general order on polishing and maintaining duty boots, please, bear with me; I promise you will become more engaged.

Thermal imaging is not new to law enforcement. Large agencies have been mounting TIs on aircraft for years, using them to assist in surveillance and pursuits. Narcotics officers used TIs to identify potential indoor marijuana growing operations, at least until the Supreme Court placed some restrictions on TI use in the 2001 Kyllo v. U.S. decision. Tactical teams use thermal imagers to gain an additional advantage, especially in low-light or smoky conditions.

The chances are that your agency actually has a thermal imager; it may be locked in a cabinet or buried in a detective's car trunk, but it's there. The goal of this column is to encourage you to find that long-lost tool and use it. As you learn how it works and become comfortable with its uses, you may consider expanding the role this technology plays in your normal operations. More frequent use will make your job easier and safer.

What is a thermal imager?

Before discussing how to use a TI, we must first understand what it is. You must understand the basic technology before you can use a TI intelligently. A thermal imager, quite simply, is a device that detects heat and translates that heat into a picture. Everything on earth generates some amount of heat; this heat is regularly emitted into the atmosphere as infrared energy. In the core of a TI, an infrared detector, positioned behind a lens, receives infrared energy and converts it to electrical energy. This energy is then transferred to an electronic board, which translates the information into a video signal and sends it to a display system, commonly an LCD screen that displays a video "heat picture." All of this is housed in a case to make it user-friendly and to protect the components from the environment (and the user). Most commonly, thermal imagers take on the look and feel of a small handheld video camera.

The thermal image is generated in tones of black and white. Black is normally the coldest object in the image, white is the hottest and the various shades of gray represent the temperatures in between. Some TIs include a switch that reverses this scheme, placing the TI in a "black hot" mode. "White hot" is the most common mode and could be considered "standard." A few TIs feature a colorization system that either highlights certain temperatures in certain colors or uses a rainbow pallet instead of a grayscale system. These colorized systems are less common, especially in law enforcement.

Regardless of the color system (white hot or black hot), all of the images are portrayed in a relative scale. That means that something that appears "white hot" in a scene is very hot in comparison to all of the other objects in the scene. For example, a person in an air conditioned room will appear hot on the TI. The same person lying in the desert on a 120 degree day will appear gray in comparison to the hot ground. Because the display is relative, the image requires interpretation. This is a skill that is learned and developed over time with training and practice.

Note that colorization systems are different. They can be relative, like the black and white systems, or absolute. The absolute color systems assign certain colors based on estimates of the objects' temperatures. These are complicated systems to use, as their accuracy is affected by many factors outside the user's control.

Infrared Principles

Remember, the TI detects infrared energy, or heat. Heat can move through the air, such as when a campfire begins to scorch the tree limbs overhead. The flame doesn't actually reach the branches, but the convected heat rises and begins to affect them. Heat can also move through solid objects, such as when a metal rod is heated at one end, and then begins to warm at the other. Lastly, heat can radiate out, such as from a fireplace, when the heat moves into the room. The manner in which heat moves, as well as the amount of heat in a particular environment, generate the differences displayed on the TI.

Like light, infrared energy is blocked by most materials. However, because infrared energy is a different wavelength from that of light, it does not pass through the same materials. While glass, water and certain plastics are transparent to light, they are frequently opaque to infrared. Conversely, smoke, certain plastics and specific metals that are opaque to light are transparent to infrared. In general, however, both energies are blocked by most materials. As a result, whether using our eyes or a thermal imager, we will "see" the surface of the object we are viewing.

When you use a TI, it is important to remember that you are viewing a heat picture, not an x-ray image. Because certain materials absorb heat, and therefore emit heat, at different rates, sometimes a thermal image will appear to "see through" a surface. In reality, it is seeing temperature differences. In Image 1, it appears that the TI is "seeing through" the drywall of this room and seeing the studs. In reality, the studs are causing the drywall to be a different temperature where they contact it, making the void spaces between them slightly warmer. This temperature difference is detected by the TI, showing you where the studs are located. But you are not "seeing through" the drywall.

In Image 2, we see a clearer example of how certain objects block or absorb heat differently. This officer is wearing a ballistic vest that is blocking his body heat, making the surface of the vest relatively cool compared to the heat transferred to his shirt. Additionally, his head is clearly the warmest portion of the image.


Prior to using a thermal imager, we must understand what it does and what its limitations are. The fact that a TI generates a "heat picture" for the officer to interpret is the basis of all of the thermal imager applications. Rest assured that you have endured the majority of the technical stuff. Please return next month as this column dives into real-life ways you can use a TI on the streets to make your life easier. After all, no one wants to read another general order on polishing boots.