Thermal Imaging: Facts vs. Fiction

How do thermal imaging cameras work?

Thermal imaging cameras, also called infrared cameras, detect the heat given off by an object or person. Thermal imaging cameras have lenses, just like visible light cameras. But in this case the lens focuses waves from the infrared energy present in all objects onto an infrared sensor array. Thousands of sensors on the array convert the infrared energy into electrical signals, which create a video image. The infrared camera measures and displays a “thermal profile” of objects in relation to the temperature of surrounding objects. So a person, warmer than the surrounding air, appears “white” while the cooler surrounding air or buildings will appear in varying shades of gray.

Can thermal imaging cameras see through exterior walls into houses?

No. These cameras only “see” heat as it radiates off of an object. It may “see” the heat coming from a house, but it can’t see into the house because the camera picks up the house’s exterior thermal image first. In fact, the thermal imaging doesn’t even see through glass because the glass has its own thermal profile.

Thermal imaging has also been used to improve energy conservation. Infrared systems have been used for years to monitor homes for heat loss to spot gaps in insulation.

Can thermal imaging see through clothing?

Not really. Although, if, for example, a suspect had a gun under their shirt, its exterior area would appear “cooler” to the camera and indicate to police that someone may be carrying a gun.

How is law enforcement using this technology?

In a number of important ways. First, it helps police officers stay safe by spotting suspects hiding in bushes or in dark alleys — in fact it can “see” someone hiding behind an object like a box or trash can if that person radiates enough heat to cast a thermal image around the object.

It assists police in pursuit. Thermal cameras can see people running in the night, even through the cover of trees. These cameras are also used to identify a recently driven car (by the warmth of the hood), or in some cases even the warmth of the skid marks left by a fleeing car.

Thermal imaging cameras are also used for evidence collection. The technology can help police officers spot an object a suspect has discarded while being pursued, or gather evidence or uncover situations of evidence tampering at a crime scene.

After executing a search warrant, police may sometimes use these cameras to look for objects hidden in interior walls, like drugs or money. These objects act like insulation in the walls, and may produce a different thermal image in that section of the wall compared with the surrounding wall space and studs.

Public safety officers such as search and rescue personnel and firemen also use thermal imaging. Many fire departments use hand-held or helmet-mounted cameras to see through smoke to find victims, or to see “hot spots” in walls before a fire spreads.

Are these cameras like the night goggles used by the military that we sometimes see in the movies?

No, those “greenish” images are really “image intensification” technology. These cameras, which consumers can buy for a few hundred dollars, amplify dim available light to simulate daylight. Thermal imaging cameras are more expensive and don’t rely on visible light. Instead, they produce a “thermal profile,” which highlights the temperature differences of objects.

Shedding Light on the Dark

Using infrared imaging technology can help law enforcement professionals stop crime before it happens. A thermal imager measures very small temperature differences that can’t be seen by the naked eye, allowing you to see in no- or low-light situations.

It takes those images and creates an infrared picture — a picture that law enforcement can use to catch criminal behavior. That’s a powerful ability, and one that the Law Enforcement Thermographers Association (LETA) recognizes. LETA is a non-profit law enforcement association founded as an infrared training organization for law enforcement ( The association has sanctioned 11 infrared imaging uses for law enforcement and is reviewing more each day. Before an application can be added to the LETA list, a federal or state court must have accepted the infrared images as evidence in a case. Once it becomes part of case law, the LETA board meets to accept it as an application. The applications below are among the LETA-approved uses for infrared imaging.

Hidden compartments.

Infrared imagers can be useful in detecting hidden vehicle compartments used for transporting illegal drugs, contraband or even people. Due to the change in thermal characteristics of a surface caused by an adjoining wall or bulkhead, thermal imagers highlight structural details unseen by the naked eye. New applications are emerging every day in the use of thermal imaging, both in and out of the field of law enforcement. Watch for upcoming articles in future issues of Eye on Infrared to address these applications.

Perimeter surveillance.

In a crisis situation, one of the first operational actions is to set up a perimeter. Thermal imagers can make this perimeter more effective in containing and apprehending the suspect, or detecting and preventing unwanted intruders from invading a restricted area. Similarly, highly restricted facilities, such as correctional institutions or industrial plants, can monitor perimeters night or day with the aid of an infrared imager.

Marine and ground surveillance.

Infrared imagers can be used for nighttime navigation on bodies of water to locate and track a variety of vessels. The ability of an infrared imager to see in total darkness allows investigators to conduct surveillance undetected and without violating citizens’ fourth amendment rights of privacy.

Structure profiles.

One of the earliest uses of infrared imaging in law enforcement involved structure profiles during indoor marijuana cultivation investigations. Indoor growing operations require the use of high intensity growing lamps which produce large amounts of heat. This heat must be exhausted through the building’s structure to maintain an acceptable air temperature for the marijuana plants to grow. The exhausted heat will manifest itself in several ways: through exhaust vents, attic vents and exterior walls. Infrared imagers see this abnormal excess heat and provide an additional element of probable cause.

Officer safety.

During ground operations (particularly at night), infrared imagers can be used to locate threats such as hidden suspects, guard dogs and dangerous obstacles. Not only can an officer locate these dangers, he/she can see without being seen. In addition, infrared imagers can see through visible obscurants such as dust and dense smoke. When approaching a burning building or vehicle, an officer can quickly scan through the smoke to see the extent of the fire or to look for victims.

Disturbed surface scenarios.

When a surface is disturbed or altered, the infrared characteristics of that surface are also altered, even though it may look the same to the naked eye. This principle can be applied as a nondestructive method to survey walls and floors for hidden compartments that may contain contraband. Turned earth also radiates differently than compacted soil, which allows infrared imagers to locate buried items because of the change in radiance of looser soil versus packed soil.


Pollutants such as oils, chemicals, and waste matter emit or radiate heat differently than the soil or water around them. Infrared imagers can track these pollutants back to their source. Airborne emissions from illegal night-time burning operations can be monitored; water pollutants can be traced upstream; and dump sights can be covertly monitored in total darkness, resulting in the arrest and conviction of the violators.

Flight safety.

Aircraft-mounted infrared imagers are effective tools to enhance the safety of nighttime flight. Hazards like power lines and unlit landing sights can be easily identified with an infrared imager.

Fugitive searches and Rescue missions.

Animate objects, such as human beings, are excellent radiators of heat. As a result, infrared imaging can be used in search and rescue operations to cover large areas quickly and accurately with less manpower. Searching for a person can be conducted efficiently during darkness or full sun light using an infrared imager. Infrared imagers are excellent at finding people and animals hiding in foliage because imagers see the heat radiated from the subject, not just the light reflected off of the foliage or person. By locating a fugitive with an infrared imager, officers can apprehend a subject without giving away their own location or without blind sweeps involving many officers.

Vehicle pursuits.

A vehicle radiates a lot of heat; both during and after its use. The heat shows up not only from the engine, but also from the tires, brakes and exhaust. Using an infrared imager, a police aircraft can track a suspect’s vehicle easily from the air, even if its headlights are turned off. Likewise, a patrol car, using a vehicle mounted imager, can track a suspect’s vehicle in a crowded parking lot or remote area by detecting the heat of the recently driven vehicle.


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