Thermal Imagers on Patrol

A not-so-new technology is expanding from specialized units to patrol officers.

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.

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